Comment So what? (Score 1) 194
UDIDs are commonly used in order to estimate how many users an application has, especially on applications that don't require people to register an account.
Tons of web sites and ad servers are also sending cookies for this very purpose. It's not bullet proof, but it's better than nothing.
UDIDs can be also useful in order to block users (spammers, people sending illegal content, etc) on social networks, as it's more difficult to buy a new device that it is to create a new account.