everyone asks that! And not only for me! I've found 1 or 2 posts when googleing for this. In them other people ask just the same for the requesters (yes! there're other people looking for something like that around the world).
Well I can imagine some uses for it, but probably there're others. 3 of them are:
1. I've already seem (and worked for) some companies that the only way to connect their intranet/network is by dialing up. Due to security reasons they do not open their network via internet/VPN.
2. With only one PC and one broadband you can this way open lots of dial-ups. This can be used for simultaneous in/out connections to lots of other PCs/networks.
3. (the one I'm more interested now!) We can make money!
Here in Brazil there are some free internet service providers that in fact they pay for us just be connected to them (in their business model they make money with the carrier interconnection). And now they pay more than I currently pay to some VSPs to call Brazil! So if I can open 5, 10,....slow dial-ups from only one PC to these ISPs I'll make some money (what they pay less VSP rate that I pay).