What’s with the rapid response response and why do we need it?

For more info, view the related article on top. The letter just differentiates it from a software update. The Rapid Security Response takes very little time to install, compared to the software update.

It is an Apple security patch. Apple has not elaborated on the particular security issue.

It's just a software update with a sexy name. If you are already running the latest version of everything, then it is a reasonable assumption that you'll drop everything to install the update. Isn't that what everyone is doing these days?