Why do some API providers require an API key?

Question

Several web service APIs have you sign up for an API key. For example, UPS Web services requires a key, which is included in calls to their service -- In addition to the use

Answer 1

Usually it used to get stats on how much application performing queries to API. I think asking username/password with API key is ambigious in some cases, but it is a way how it is implemented - so we can't do something with it.

They ask for API key because you could have more than one API under same account - in case you have more than one site which are use same API.