You don't necessarily need them. Also, a band might not have a keyboard player, but in the studio, keyboard parts just happen. Maybe on the spur of the moment or whatever. It could be just a simple synth part. You have to suck for a while before you can actually play guitar.
Keyboards are different.
Yes, they're very difficult to play well, and yes, it takes years to learn how to play, but a keyboard is one of the easiest things to walk up to and get a cool sound. Almost anybody can use one to add a cool part to their record, so a lot of us get hired after the fact because we're needed to support the record some band just made. Take me, for instance. Therefore they needed someone to cover that part live. I suggest sounds and parts and licks.