I would say that for the most part keys are there to add a sense of fullness and depth, not unlike the role of a rhythm guitarist. Consider how many times a keys player is asked to emulate a sting or horn section. Certainly solos are warranted, and there are genres where keys certainly take a more up front stance, but overall, I think they're meant to add the flourish and sparkle that makes a song really breathe and live.
Think about what you've heard. There are songs; arrangements of songs where you know there's an organ or some such in there, but it's barely audible. It adds substance without being obvious. It's sort of like how they used to regard the bass: "It's to be felt, but not heard". Obviously, that rarely applies these days, but I think you'll see my point. There are other songs where the keyboard sound is far more prominent, and often critically so. Could you cover Stevie Wonder's "Superstition" without a very in-your-face clavinet sound. No you could not. Could you do "Roundabout" without a sweaty B-3? Nope. You have to be flexible, adaptable, logical, responsible, sensible, reasonable, and smart. Be a musician first; remember that even if your contribution to the song may seem minor, where the music is concerned there are no small parts. How many times in school did I sit there counting who can count how many measures of rest before it was time for me to add my 2 cents worth in band? If you're a REAL musician, then you're an ensemble player, and you have to do your bit, even if you find it dull. 'Tis y' job, mon.
Jah love to all.