If all you care about is the structured PvP, then there isn't a point.
Leveling in PvE does nothing to make you stronger in sPvP (nothing will make you stronger there; they've tried to make the playing field as level as possible), so if that's the only aspect of the game you're interested in, then no, there isn't much of a reason to level up in PvE.
However, there are other reasons to level up in PvE:
seeing more content: the first dungeon isn't accessible until level 30, the story runs all the way to level 80, and at level 20 you'll be able to survive in less than half of the zones, so if you want to see everything the game has to offer, you'll need to hit the level cap
becoming stronger in WvWvW: the other PvP format, world vs world, will scale you up to 80, but it doesn't do it in the same way that the PvP arenas do; leveling up in PvE will absolutely make you stronger in WvWvW, since it will unlock trait points, skill points, and help you find better equipment (you can, of course, level up solely in WvWvW if you so desire)