The idea behind that rule is that a striker should not just hang around in front of the goal.
That may lead to just long passes to the front, and not a nice gameplay overall.

Therefore the most front a striker can be, should be as far as the last defender. (not exactly the rule).

This could’ve been the rule, but it is just defined a little different: The most front a striker can be, is as far as the second last opponent (since there is usually a goalie).

Now, when mustn’t a striker stand there: Not never, but at the moment of a pass towards him.

As you are allowed to hang around without participating, sometimes you still have an influence on the game by interacting. For example you dodge a shot towards the goal. Now the goalie does not know, that you will dodge and has to consider both scenarios. In this case, even if you did not hit the ball, or even did not move for dodging, you have intereferred, and if you were offside, at the moment of the shot, it will be offside.

I guess that is most of it.