Sometimes you don't have a bass player. It happens a lot. Sometimes you perform alone, and want the guitar to sound bigger, and more full.
Here's a really cool Kickstarter campaign going on right now. This guy works for Steve Vai, and has come up with a pretty cool invention. It's called A Little Thunder.
It's a guitar pickup that fits in a regular humbucker slot on the guitar, and has sort of a stacked pickup to give a humbucker sound in a single-coil space, and another couple coils for just the low E and A strings, to send to bass sound.