Standing On Your Own Three Feet
By definition, a tripod has three legs. That means it also has three feet where those legs contact the ground. Don't make the mistake of only paying attention to the what's on top of your tripod.
It's time to take a look at the humble tripod for a moment. A human photographer with two legs can hold a camera reasonably steady, but they could find themselves with a tendency to shift position, even if ever so slightly. You may be wondering why photographers sometimes choose to use a monopod in their efforts to improve stability when they themselves have more legs than a monopod does. But a monopod does do a better job of preventing vertical movement than a photographer can when hand-holding their camera. And since a monopod would quickly fall over if the photographer using it doesn't keep a hand on things during the shot, stability is improved further via the loose coupling of photographer to monopod. But if you tightly couple those legs together, you get a tripod. Two legs plus one leg equals three. In the quest for sharp images, you need a tripod.
Three legs are the minimum necessary to achieve true stability. Any fewer legs and at least one axis of movement would remain possible. No additional stability would be possible by adding a fourth leg. All that would give you is a constant pain when adjusting your "quad-pod" on uneven ground.
Both as a means of decreasing weight and to facilitate adjustments to the height of the tripod head mounting platform, tripod legs are generally hollow, with sections that collapse into the next one in line. My favorite tripods are those made by Gitzo, but there are other brands in common use that are less expensive. Surveyor tripods have legs segments that slide side by side rather than into each other, but designs of this type are inherently more bulky and heavier than what most photographers use. I actually have a carbon fiber tripod built on the model of a surveyor tripod that I bought many years ago to try out, but it really can't compare with a well-made Gitzo in real world field use.
The usual means of terminating a tripod leg is to cover it in simple, hard rubber endcap, much as you would find on the end of furniture legs. With furniture, such caps prevent scuffing your nice hardwood floors. While this would be equally true for tripod legs, I and other predominantly outdoor photographers don't work on hardwood floors that often.
Still, something has to be on the end of those hollow tripod leg tubes. Hard plastic feet are used for some tripod designs, but for outdoor use there isn't much that can compare with solid metal in terms of durability and stability. While rubber can dampen vibration, it won't prevent it. Whether it originated from the shutter release, reflex mirror slap, a gust of wind, or just a moment of carelessness on the part of the photographer, any slight jiggle to the camera would vibrate across the mass of the tripod/camera system. Metal feet allow the mass to couple more tightly to the ground below. Metal won't flex, vibrate or compress.
Metal feet can be found shaped as both claws and as spikes. Claw designs dig into hard rock surfaces quite well, but spiked feet work better when a layer of dirt covers the bedrock beneath. Typical spikes or claws are made from stainless steel. For many years, I used a prototype spiked foot made from solid titanium by Markins in South Korea. Perhaps a tad extravagant, but titanium is both lighter and harder than steel. Those feet were basically indestructible. Unfortunately, they were also held into the ends of my tripod legs with permanent epoxy, so when I upgraded tripods, I was forced to leave the Markins spikes on my old tripod. My new Gitzo spikes are stainless steel, but have worn well so far.
While spikes or claws are ideal or outdoor use, they are horrible for shooting indoors. Their very ability to dig into bedrock on the side of a mountain would severely damage hardwood or tile floors. Some tripod foot designs attempt to solve this by designing the spike so that it can be screwed further into the surrounding foot until it becomes recessed and thus no longer able to scratch delicate surfaces. If you have such feet, check them closely. That same hole may provide an opening for mud and other gunk to work their way into your tripod leg. Other spike designs address the problem by providing caps that screw on overtop of the sharp metal. So long as you don't lose them, this can be a good option.
All this talk about feet brings me to another potential problem: leaks. Encountering mud and water are common occurrences when shooting outdoors. Place a tripod foot in some mud and any screw threads it may have will eventually become fouled and stuck. Place that foot in water, and those screw threads can provide an avenue for water to seep inside the tripod leg attached to that foot. Salt water in particular can have a corrosive effect on almost any material. Salt deposits can be hard to remove. Eventually, the legs of your tripod may not collapse smoothly into each other anymore.
Examine your tripod feet. If they screw on or screw in, see if you can remove them completely. What do you have when you do? Is there a hole that goes all the way into the leg? Such configurations are not uncommon. You can help seal things with a small amount of heavy weight bearing grease applied to the threads or perhaps the addition of a rubber O-ring to the foot to seal out water when the screw is fully tightened. When shooting in sea water, you may even want to consider taping a plastic bag or sleeve over the bottom leg segment. The exact course of action will depend of course on your particular foot design and shooting environment.
Working on snow can create yet another problem. Standard tripod feet of any design will sink right down in. The deeper the snow, the worse the problem. People have the same problem when standing on snow of course, and the usual solution is to wear snow shoes. Thankfully, you can buy snow shoes for your tripod legs too. The ones Gitzo makes are marketed under the name of "Location Shoes" and could theoretically prove useful in deep sand as well as snow. For the price, you can't beat them, and they should work on most any brand of tripod foot, not just Gitzo.
One final point worth conveying here is the importance of keeping your tripod legs and feet clean. It can be easy to focus most of your attention on the tripod head at the top of your tripod, but the feet at the bottom are also important. Keep them clean and they should serve you well for many years. If you do much hiking, you know the importance of keeping your own feet healthy. The same basic principal holds true for your tripod feet.