kareila linked the FAQ, but the paid account benefit I get the most out of personally is the ability to search just my journal and also search my comments. It's great for "I know I've posted about this thing sometime in the past 20 years, but I was really lazy about tagging for a while". (Runner up: the ability to read my reading page by what it looked like on specific dates in the past, for when I forgot to check things for longer than the reading page runs back for again.)