Your personal computer will inevitably have sensitive information and content which you wouldn't want anyone to look at. You also don't want people messing around with your settings and fiddling with your files.
If you forget to lock your Mac when you move away from it, Keycard is an app that could help. It's a neat little app which connects via Bluetooth to your phone or tablet. It'll then setup a proximity, once one of your devices leaves that proximity, it'll lock the Mac.
So if you quickly needed to pop out you'll most likely take your phone. Once your phone is far away enough from your Mac it'll lock itself, denying access to anyone who tries to get on it.