Safe to uninstall hardware

I have been getting instances of the computer re-booting when disconnecting
a USB device and have been told that I should use the "Safe to uninstall
hardware" (I think that's the right phrase) before disconnecting.

Some sources describe USB devices as "hot swappable". This suggests that I
should be able to connect and disconnect USB devices when required without a
problem (as long as they are not being written to at the time).

This all seems to be contradictory. If they are hot swappable why is there
a need to use "safe to uninstall hardware"? If using it prevents some kind
of surge why is there not a "Safe to install hardware? This is compounded
by the fact that the "Safe to uninstall hardware" icon isn't always
available on my computer.

Bill Ridgeway