it's not even the hat, it's anything
even if it was a little number in your profile that incremented and said "TOWER GUILDS COMPLETED: 1"

it's just human nature, at least for gamers, to want some sort of badge for things that they've achieved
I don't even wear my hats from tower guilds and I have 4 of them, but it's cool for me to have just to show off and remember the guild by

you and I both know that if they added hats for sparring and PKing, it'd bring more activity to both
the PKing part would indirectly affect towering as well, maybe it'd make towers a little more of a challenge by spawning a few new tower/PK guilds