It's for graphic annotations. Largely used when exporting to image or .pdf files. Imagine you have a complex map and want to add a one-off text box and arrow. Just to this map in Pro and no where else. Map Notes let you do this easily and you do not have to clutter your geodatabase.
Annotation or vector feature classes definitely have a lot of advantages. If I was adding a lot of manually created callout labels and wanted them to be used across several maps I'd definitely look at annotation feature classes over Map Notes. But there's more setup and administration involved, Map Notes are definitely way easier.
They are also familiar to ArcMap users. Lots of users would add text directly to a map and wanted to be able to do the same in Pro.
TL;DR = Map Notes are graphic elements. More contained and much easier to use than setting up feature classes. If you need quick and dedicated to one Map, for exports, map notes can be OK. For large scale use, annotation feature classes probably better.
