It isn't, necessarily...don't forget, they want to sell us stuff! It depends on your skin-type, and what's in your foundation--and perhaps the time of year...use your own judgement. Newer products have moisturizer as a component--you might want to try them!
There are a couple of reasons depending on what type you use. The one reason is so your skin does not dry out and makes your foundation look more even when you put it on. I use a sunscreen moisturizer which I love--so that adds sun protection under your foundation. I also use a favorite liquid foundation that is also has sunscreen in it. Double the protection!