brew update is being called over and over again in most use cases - if only to check for example, if the latest openssl release patching a critical exploit is out yet. (of course on the mac openssl is packaged with the system, but if you are using the brew version, or are compiling other software against openssl from brew, you'll need to check for updates diligently)

It is also being used in scripts, etc. Since from the user's point of view it's a no-op if there are no updates, there is no reason not to do it on a schedule.