You would think that Google being a huge company would have good solid reasons for everything they do. But building software (and search engine websites are software) involves billions of small decisions taken by each engineer involved. It's everything down from "should I name this temporary variable 'i' or 'n'?" to "should the query time be displayed?" and "what format? milliseconds or fractions of a second? should it be i18n:ized?"

My theory is that it just happened because some engineer decided to add it a long time ago and then noone found a good reason to remove it.