beyond being sometimes useful and descriptive of the context, you can just write fucking Anything in them. you can be as normal or absurd as you want with them. you can write something birthed from the depths beneath severals layers of irony, and sometimes you might even find some other poor soul has deigned to compose the same cursed string of characters you felt compelled to assemble yourself (okay maybe this is also tumblr, but I never really used tumblr, potentially because of how maligned it seemed to be when I was a dumbass who was convinced some rigid notion of propriety was important?...still there's a tangible mystery and tension that can be felt when you stumble across a tag that has less than like, 9 chosts, down to and including 0)
