If you are new to weight lifting putting on muscle is easy at first. But after a while you may notice that your gains begin to slow down or even plateau completely.
That's because your body very quickly adapts to the new stresses it has been exposed to in the gym. Your muscles become more efficient at performing the moves you do most often, which means that they don't have to work as hard to do the same amount of work. In short, performing the same exact workout week in, week out isn't an effective strategy to keep building muscle.
Plus, doing the same session is really, really boring. You'll never add muscle if you don't go to the gym because your bored with doing the same old session.
Luckily, there's a very simple solution to this problem. If you want to keep your muscles growing, you must keep them guessing. And the that means you need to regularly shake things up by introducing new exercises that will "shock" your muscles into growth because they are unfamiliar with these new moves or variations and the new mechanics they involve.
Sometimes this will mean introducing a completely new move that hits the same muscles. At other times it will simply mean tweaking a move slightly, by say changing your hand position, the angle of the bench, your grip on the bar, or standing up instead of sitting down, etc.