Not sure about using it as a tablet, but if that is possible it would surely say so, and how, in the instructions manual.
Using it as a modem, yes - does exactly what any modem does, enables the user to go on-line. Why? well partly because some people do not have a land-line or cable, so they could use the mobile phone instead to get a pc on-line. Especially overseas, like in the desert or in the jungle - those those are extreme cases. I have a friend in Tanzania whose uses his mobile as a modem.