Footwork

The idea behind having one foot slightly further forwards is so that you can push off both left and right, as well as forwards and backwards. If you had your feet square to the net, you would only get side to side, not forwards and backwards as well.

For this reason, many professionals have their feet nearly parallel (neither foot is forward) when they are anticipating a fast attacking shot (fast drop or smash) by their opponent, and will have to cover the court sideways quickly. even in this scenario however, a player often has one foot (usually the racket foot) slightly forwards.

Having the non racket foot forwards is most often used when you are certain your opponent cannot attack you, and you anticipate a lift i.e. you just played a very tight spinning net shot in singles, and you expect your opponent to lift. With the non racket foot forwards, it is normally very tricky to reach the backhand rear corner, and as such isn't used much. This is used exclusively by top professionals who are returning serve. Why? Your opponent cannot attack you or force you to move side to side, but you are ready for both the net and a flick serve.

Racket foot slightly forwards (right foot for right handers) is most common. It allows good movement to all four corners (although maybe the forehand rear court is trickier). In particular, playing shots from the rear backhand corner is particularly easy with this foot positioning. This is most often used as a neutral stance or when defending in doubles, allowing unrestricted arm movements to defend both backhand and forehand sides easily (non racket foot slightly forwards gives difficulties when defending the backhand side in most situations).

Racket foot very far forwards (almost standing sideways on to the net) is referred to often as an "aggressive" stance. In this position, most often seen in singles after a net shot, the player is ready to leap forwards for a net kill, or to spring backwards to attack a lift (usually using the backhand rear court footwork, regardless of where it goes). This is only used when you are fairly sure you won't have to move quickly side to side (as you won't be able to move easily in those directions).

Hopefully that gives you a feel for what sort of things are the cause of different foot positions. No doubt I have missed some - what I have said are not the rules, just what seems to be most common amongst professional players - in particular in singles.