It’s the illusion of security. People are afraid their computer will be hacked, or they’ll grab a virus, or their computer will crash, but some central repository says they’re secure... I wouldn’t want to keep thousand of dollars of virtual money on my PC, especially not my old PC that was going through death throws. Of course I wouldn’t keep it in some random website either. If I got a bitcoin my #1 goal would be to turn it into cash in my normal bank by the end of the day.
That’s why you keep a copy of your Bitcoins on some type of external storage. Something as simple as a USB thumb drive is fine. You can spread copies to several devices for redundancy.
If someone steals a copy, they can’t so anything with them as they do not have the private keys (which hopefully you made sure you have somewhere safe in a separate location from the Bitcoins) for them.
I don’t have any Bitcoins either, I just find the tech fascinating.