1. They log keystrokes years before you ever got access, giving you the ability to go back in time! Sometimes all the passwords are already conveniently logged in a file called passwords.txt.
2. Logs from human keyloggers often end up on multiple computers via the network. So if you gain access to a computer and get the users' e-mail, you can see not only the keystrokes that your target has entered, but the keystrokes from people responding. The same applies for documents on network shares.
3. You sometimes don't even need to gain access to a target computer, and can learn quite a bit by using OSINT.
4. Not only do keystrokes get logged, but general user activity like browser history, program execution, contacts, files system activity, Wi-Fi networks used, etc.
5. No malware or hardware needs to be installed, human keyloggers are FUD.