The ONLY reason to purchase a static ip from your ISP, is your intent to run
a public web site -- in which case you will also need a special user agreement
from the ISP.
btw: The ISP is your connection to the world. Whether you have dial-up, cable,
or ADSL, the ISP will control your public ip-address. Your connection to the ISP
via a router, will create a LAN with NAT support in the router. To get your
web server accessible to the net, you use a feature known as port-forwarding
to the LAN ip-address where your web server is installed.