creases are used on sub D's if you want an edge to look sharper but not be 90 degrees then use 2 or 3 partial creases along an edge, if you want a sharp transition between one area to another say like a crease you would find on a hood then you use full crease along an edge.

If you converted to sub d's and all your corners got smoothed out say along a window then you could select the vertices on the corners and apply a full crease this would then pull them tight

Cheers
Rob