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
Originally, Blogger required you to apply for an API key (a la Google Maps) and used it to restrict access to the API. As Blogger evolved into Metaweblog, the requirement for the API became less important, and Blogger no longer requires you to apply for a key. As noted by others, it can still be used for tracking purposes.