Have you ever found yourself in the position where you have to update your CV? It's not fun if it's been a long time since you last did it. If you maintain a profile on Careers and update it as you go, it's trivial to export an up to date CV when and if you need one.
It also allows for passive job seekers, people that aren't looking for a job, but would be open to being contacted by potential employers. It lets them leave their proverbial hook in the water, if only out of curiosity for what they might catch.
I use it just to keep my stuff in one place and up to date on a semi frequent basis. There are times (rare, but it happens) when a client of my company wants to see the CVs for everyone that would be working on a project. In a few clicks, I can export a copy of my Careers profile right to PDF and be done with it.
And well, I like checking my stats from time to time. I really enjoy artificially inflating my ego by noticing that more people found me interesting enough to view my experience.