Imagine a straight line straight up from your front hub. That line would be 90 degrees from zero.
Less angle means slower steering, better tracking and stability.
Same deal with a straight line up from the cranks. The amount of angle of the seat tube also effects steering and center of gravity. Low seat angles put you over the back wheel and work good for tucking and wheelies. Also to how the bike feels in corners and when cranking. If you increase the chainstay length the bike will handle slower, and be more stable at high speeds.