I wonder what are the usecases of having a web server on a Raspberry Pi, because i believe this type of web server can be used without the internet.
if you know some usecases can you share your ideas?
You are correct that the Pi need not be connected to the internet to be useful, however, that does not mean it can't be useful nor does it prevent it being connected to the internet. There is even a company in Europe (Germany if IIRC) that will host your Pi in their data center and make it accessible via the internet.
Use cases include:
Note that some of these would require an internal network or LAN to be useful, nor is this an exhaustive list.