Why do i realy need the accelerometer??

[KenWittlief]

Since gravity is always present, and aways vectored straight down, you can also use the accelerometer as a tilt sensor

to tell you if the robot is tipping (falling over or on the ramp), or to measure the angle that an arm is tipped.

you can also use it to register impacts - if your robot hits something in auton mode, the accelerometer will register a sudden change in acceleration.