Disk encryption isn't really about online security - but rather offline security.
OK, if you put your personal data to an encrypted folder and don't mount it, they'd be protected against malicious code, should you get infected in between - so that would be online security, but that's not the main scenario IMHO.
Disk encryption is there mainly to protect your data if your computer gets stolen (or if anybody has a physical access to it).
Regarding the full disk encryption - the difference is in the system partition (which you cannot encrypt otherwise). So, if you encrypt the whole disk, you also protect Windows registry, the system folder... and of course anything you have stored or installed on your system partition. It's probably up to you to say whether you think you might have any "interesting" information there.