At the moment when the ball is passed forward to the striker, there must be at least two people (i.e. the goalkeeper and one defender) between that striker and the goal. If not, that striker is deemed to be offside. This prevents people from just hanging around the goal waiting to thieve opportunities when everyone else is down the other end of the pitch.
But surely its the strikers job to be thieving opportunities and the defenders to be defending. How is it the strikers fault that the defenders arent doing there job?
It promotes better football. It would just suck if one goalkeeper could hoof the ball right up to the other end where a striker has been waiting by the goalposts since the kick off. Unless of course you're suggesting that two or three of the defenders should permanently station themselves on the goal line to prevent this.
The only difference was that you said there needed to be a defender between the striker and the goal when they scored, whereas it only matters when the ball is passed to the striker. If they're any good they'll run very fast and leave the defenders behind before they shoot. >_>