Why do we boil water to cook food? It's not actually because there's anything magic about the boiling of water, or that the physical process of boiling in particular does anything. Usually it's because we want a constant-temperature heat bath. Say you are boiling vegetables. You boil water, and you know that water is at 100 degrees. Water actually cannot get any hotter than this--it stays at that temperature or it becomes steam and leaves the pot. Then you put the vegetables into the boiling water, and therefore you know that they are in a 100 degree environment. Then, you know you need to leave them in there for however long--let's say five minutes.
Suppose, though, you were at high altitudes and water boiled at 95 degrees. Well, now when you put your vegetables into boiling water, they are only in a 95 degree environment, so the cooking time has changed. Your recipe no longer works correctly, and you will have to boil the food for longer.
Or maybe not, actually. The other possibility is that you are putting food in an oven, say something that's supposed to be 200 degrees. In this case, the water that's probably in the thing you're baking actually keeps the food cooler for longer. It reaches 100 degrees and will stay there until it boils off. However, if it boils off at 95 degrees, then again your cooking parameters will have changed. Since the water boils off faster, your dish spends more time at higher temperatures and can burn.
The common factor here is that boiling water is very energy-intensive, more so than just getting it to 100 C. It is therefore quite easy (and common) to get water to 100 C and have it stay there for some time, and we use this in cooking to calibrate our recipes. If pressure changes, than all the temperatures we assume to hold in cooking change, and procedures may need to be adapted as well.