#1
I'm just really curious. I've always wondered what the 9 volt battery in the back of a guitar does.

Sorry! Just wondering.
#2
I am by no means a guru at this but they are for active basses. It means Either the pickups will have a boosting sound, or there will be an EQ on your bass for quick adjustments to your tone or both.
#3
mine basically completely runs on it. I have an Ibanz SR506 and when the battery starts to die (which takes a long time, don't worry) the bass will start to sound like its dying and sound like crap, then pop in 1 new 9V and it's back sounding like you just bought it yesterday!
#4
If your bass has a battery in the back, it means that it is an active bass. Active basses use a 9 Volt battery to help boost the tone and quality of your sound. It is completely opinion on wether or not active basses are better than passive (doesn't use a battery) basses. You won't need to replace the battery for about a year, so thats $2 a year out of your pocket, not much.
#5
My Opinion, it's worth it, you can customize your tone with treble,mid,bass however you want. It really helps you get the exact sound you want.