Welll that's what I'm asking I mean. Why should I be using one, and in what cases, like all the time, etc.
If I don't use one, how exactly is it dangerous, how can someone get my IP, etc.
One reason is to defend yourself against a 'honeypot' website, offering services in bitcoins in order to capture your IP data for some future government crackdown on users. My main client runs completely over TOR, to the same end, and doesn't respond to connection requests from unknown clients. It's competely "quiet", and wouldn't even be counted in those network connection maps. This is a bit paranoid, I admit, particularly since I can be identified by this very website forum's logs; but it's astronomically unlikely that anyone is going to be able to use any kind of network analysis techniques to figure out my IP or bitcoin addresses, no matter how many compromised nodes that they are willing to pay for.
Another would be to insulate your client's machine from a direct attack, in the event that some future hacker figures out an automated or simple way to identify bitcoin nodes by IP, and then attempt to crack in and straight up steal your wallet.dat file and/or install a keylogger to figure out your wallet's passkey. Security through obscurity is untrustworthy, but it can still be very effective in practice.