It can be a useful tool against fraud etc, eg when i log on to Internet Banking for example, my bank can tell if Im logging on with a 'different' IP address from normal and double checks that its really me.
I think ebay do it too.
It is a bit like asking what can you use your phone number for. You need a phone number to use the telephone network, but you cannot actually "use" it for anything.
You need an ip address to connect to the internet (every computer on the internet has an ip address) but I dont think you can actualy DO anything with it.
And if your computer uses DHCP (most do) then your ip address could be different each time you log on.
Unless you're paying your ISP a great deal extra for a static IP address then it will vary as VHG says everytime you log on and unless I'm mistaken, the very fact that it isn't constant makes it the more secure method as a static one would be completely predictable.