What is "garage band"and why do I need it?

From https://itunes.apple.com/us/app/garageband/id408709785?mt=8

GarageBand turns your iPad, iPhone, and iPod touch into a collection of Touch Instruments and a full-featured recording studio — so you can make music anywhere you go.

You don't need it. It's just there if you want to use it. You can remove it you want to.

Garage Band is an app that comes with your phone when you first get it. It's not that you need it, its that it is there for music lovers who want the freedom to create beats on their phones.

No one here could know if you need it or not. I would guess that since you do no know, then you do not need it.

You can look in the app store and read the description.

GarageBand on the App Store

Here is a short user guide: http://help.apple.com/garageband/iphone/2.0/index.html