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

Most of the time it is to monitor how developers are using the web-api. If they somehow disagree with your usage of the api it provides a means for them to shut it/you down without hurting the other users. And the statistics per user/app are always valuable.

I've used the flickr api - in that situation the key is yours, but the login data might be those of people using your app, so the api key is the only way to differentiate between the apps.