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.

What it does is take the notes from those two lowest strings, and drop them by either 1 or 2 octaves, depending on how you set it, so that you'll instantly have bas playing along with your low strings.

Or you can use it to just add some serious bottom end to your tone.  Check out the video, and see if this is the pickup for you!  Pretty cool idea.