Most of the previous answers are about finding issues with the code (whether just unnecessary tags or actual errors).
But it can also be helpful for things that are both needed and working, for example detail/summary tags.
In the RCE, without the indication in the bottom, the only thing you'd notice is the triangle ▼ (which could be just plain unicode as I used here).
When the first row (summary) is selected, we get:

When the second row selected:

In short, it may be helpful not just to indicate that something is wrong or unnecessary, but that the representation of the text will likely be different than what the RCE edit view shows (I have another use case with a content-box in a page, but that'd take longer for me make a minimum example to share).