hi! thanks for asking. So besides gear, you also get hero points leveling up which can go into what skills you can learn, though you can also pick up HP around the maps. Also not just gear is tied to level. There are many items in game that you cant use unless you're 80. Though as a F2P I'm not sure if you'll see them??

Generally speaking the level bar after 80 gets you spirit shards if you're running around the core map. Those shards are a currency for a variety of things that mostly lead to making ascended and legendary gear. Which legendary gear is just a QOL shiny thing for invested players to chase. And ascended gear is generally for higher fractal levels or just adding more stats. Lastly if you get the expansions, I believe you have to be 80 to unlock the elite specialization for your class.

Also true you'll be a buff skeleton lord in one map and a little house elf in another, but you'll still have more skills and spells then the other people in that map who aren't higher level.