For many problems, you never need a stringmap (in any language so JS). For a lot of problems, you do.

Perhaps the administrative software for this school wanted to display the most popular names in each class, which you'd typically do with a stringmap and many would mistakenly do with an object. That's the same problem as counting word frequencies in a sentence.

__proto__ bugs are a real thing, and it affects small and large apps (Google Apps comes to mind).