Anyone tall enough can be used as a jumper. At 8 you will get into the game a lot more than at lock. Moves from the scrum where you pick up from the base really help to mix up the play. Your scrum half will really appreciate it if you can pick up and make even just a couple of yards to set up a winning ruck if your scrum is under pressure. A good 8 will also read the game and "corner flag", meaning that he covers across behind the line and can often be in position to make a saving tackle or be there to support the wingers and full back in defence. Any back that gets caught in defence after fielding a kick will breath a sigh of relief when his 8 arrives to help out.You're not expected to be the first to the breakdowns like the flankers. Because you're probably heavier than the flankers you'll also be looked to to make hard yards in attack. A good 8 is the guy that the commentators say "what was he doing there?" because your anticipation got you to where the ball was going to be before the other guys realised it. While the flankers are charging around like crazy men you're the guy that's moving into that big space where the opposition 10 is going to kick the ball in just a few seconds. Reading the game is a huge part of 8 play and they're sometimes described as a defensive technician.
