Your IP address, which stands for Internet Protocol address, is the address your computer uses to communicate with other computers on a network. It's like a street address for your computer.
You wouldn't meet a complete stranger that you know nothing about and hand him your home address. Similarly, the idea of giving your IP number to various websites and computer users is a bad one. But through computer hacking, almost anyone can find out your IP number and use it for various purposes, none of these are good for you neither for anyone. A cookie may remember your login information, for instance, so you don't have to renter it each time you login. But large number of popular websites don't pose a single threat and only use IP lookup to protect themselves.