Hi, can anyone actually point out why, what and where the inverse matrix is used in openGL and why it is useful? thanks! 
you can use the inverse matrix M^-1 of matrix M when you want the changes that M did want to be undone.
If you want another answer then please specify your question
Ooh right thanks What about the determinant of a matrix, how is that useful in 3d graphics/games?
The determinant almost serves for calculating the inverse of a matrix.
It is used to determine whether a polygon is backfacing.
could you possibly explain how? The thoery behind it? If it’s too lengthy explanation it doesn’t matter
The explanation is a bit involved and it requires some knowledge of Linear Algebra.
I won’t get into all of that here but I can refer you to this
should you care to read up on it.
The inverse transformation matrix is necessary for vertex normal transformation.
The vertex itself is multiplied with the modelview-projection-matrix. Normals need to be multiplied with the inverse transpose of that matrix to stay correct.