Why is foundation used when applying makeup?

It does two things: it evens out your skin tone, and it makes the rest of your makeup stay on better.

Foundation is the base of your makeup. If not applied well, it could ruin your overall look. Use a damp sponge for a smoother application.

(That's a very good point, Dagny; if your foundation is not applied well you won't look good regardless of what you use on top of it or how well you apply it.

There are four good foundation application tools. Your fingers are not one of them.

First is the latex sponge wedge. These you use dry because they seem to be waterproof. I did a test with these once. I put a sponge wedge in a jar of water and left it there overnight, then put foundation on half of someone's face. Then I used a dry sponge to do the other half of her face. There wasn't any difference between the two sides. Latex sponge wedges are far better than your fingers, and they work pretty well, but there are better things to use. These are single-use sponges--they're so inexpensive you can afford to throw them away after one makeup application.

Next is the polyurethane sponge. These will absorb water, so you use them wet--dip them in warm water, pull them out and wring them out until nothing more comes out. Once you apply your makeup with the sponge, wash it in soapy water and reuse it. These are really nice, and I think they'll be the first choice in a few years--right now they're not because they're a little hard to find.

The natural sponge is better than the polyurethane sponge. These are also used wet. They've got smaller pores than polyurethane sponges do so they do a better job. They're getting harder to find because the sponge beds in the Mediterranean where these are harvested are nearly fished-out. A natural sponge is a piece of a marine animal, so vegans shouldn't use these.

Finally, there is the foundation brush. These are getting very popular for two reasons: they're reusable, and you need a LOT less foundation when you use one. A decent foundation brush will cost you $7 to $10; a pro-grade one from MAC is $34. Or you can do what the really cool kids do and go to an art-supply store; a 1" mop brush or flat brush would be perfect for foundation. (Don't blame me; they're really called mop brushes.)