As Riz said you don't have to install it globally, but it is also worth mentioning that just because you install it globally doesn't mean you can't run it multiple times on different ports while it is installed globally.
The biggest reasons to not install it globally are if you want to run different versions or don't have admin privileges or just don't like things installed globally.
You can serve as many tiddlywikis as you want with the same global install using
tiddlywiki /path/to/wiki --server port
where you change /path/to/wiki and port each time.