item level 115 gear

[Kazamoto]

Because leveling is there to teach you how to play. Basically a giant tutorial that eases you into your role.

See FATE's and Low-level duty roulette. 20-50, only doing sastasha teaches much about tanking.

Because character levels and item levels are a simplistic and visible aspect of your character's power, and it's long been an established norm in RPG to use these numbers to make players feel stronger.

What you're asking there is perfectly valid questions, and the answer would be that levels is a base design aspect of this game, that's the only reason we have it.

But like many base design aspects, it feels like it was only included to hit a check box on the list of things an mmo should have, like crafting. It's included, but you don't really need it.