Computers don't *always* have to reboot after installing software...but when
they do, here's why: the installer needs to modify a file that is currently
in use. By rebooting, the file can be modified before the program starts or
before Windows starts.
If the installer needs to have the computer restarted, it will typically
display a message saying so. Otherwise, there's no need. However, many
people like to reboot after every software installation regardless, just to
make sure that their computer can, in fact, still boot and run successfully.
This probably not necessary, but there's no harm done.