Earlier quoted context omitted.
So that it's grep-able?
Why would you grep in a scenario where a note is written because "[the]... code is functional, but if you are going to do another iteration you may want to consider the following improvement or optimization.". It's exactly meant for when you encounter it, it's not meant to be found externally.
Lowering the barrier to see such comments increases the chances of people actually handling them. Having a highlighted TODO in the code makes it much more visible even if you don't use grep.