Question about HTML message editor

Last night, I decided to try composing an email in the HTML message editor for the first time. I noticed this in the lower-left corner, and discovered that the items were clickable. I assume they are formatting indicators (“p” for “paragraph”?)

I was wondering what the point of their clickability was, because the behavior seemed to be inconsistent. I wondered if it was unintentional, or perhaps it’s that I simply don’t know how to use them properly.