Mainly so they don't look too washed out from all the bright lights, especially in studio. They also use it to look a little more tanned, flushed, or pale if the scene demands it - like if they meant to look really sick or cold or they meant to look like they've been stuck out in the sun for a long time, that type of thing.
I guess sometimes they use it to look better and hide blemishes, but i think if they really wanted to hide blemishes, they'd do it in post-production instead. And they wouldn't bother hiring someone that didn't look right for the part anyway, maybe the character is meant to be "TV ugly". idk for sure though.