The accelerometer tells your phone whether the screen should be in portrait or landscape mode. That is about it.
It has a Compass too, this tells your phone what direction it is facing. But that is not exactly helping.
Say, you are holding the phone direction North (compass), in landscape mode (accelerometer) ....
and now you want to tilt the phone (still in landscape, and your couch is still facing north) ...
then you need a gyroscope.