Imagine, you are at a coffee shop, and you decide you want to go to the restroom, you leave your laptop unattended, a hacker can walk-up to your computer, plug a USB stick into your device, it only takes a few seconds to siphon your computer data, and be gone before you get back to your computer.
First of all, never leave your laptop unattended in public places, but it never ceases to amaze me how relaxed people are when it comes to their devices. I have seen people at coffee shops, airports and other public places entrusting complete strangers to watch over their stuff while they go on a potty break. But they do. If you were a target or if there was a hacker in the same area as you are, it is pretty easy for them to do what they want when you leave your computer unattended and without them having to log in to your device.