Identity matrix does nothing. It looks like this:

1,0,0,0
0,1,0,0
0,0,1,0
0,0,0,1

And that just multiplies everything by 1 if applied to other matrices or vectors.

Read more about matrices

An identity matrix could be used as a starting point for some calculations, or if your object is at the origo of the world with no rotations, then you would apply identity world matrix to it.

Identity matrix does nothing. It looks like this:

1,0,0,0
0,1,0,0
0,0,1,0
0,0,0,1

And that just multiplies everything by 1 if applied to other matrices or vectors.

Read more about matrices

An identity matrix could be used as a starting point for some calculations, or if your object is at the origo of the world with no rotations, then you would apply identity world matrix to it.

The setIdentity method might be resetting your matrix back to identity matrix, and if your matrix is not declared inside the method you use it in, it is needed to be reset back to identity. Otherwise more and more rotations are applied on the matrix which results in the fast spinning.