Since Isidra’s explanation does not appear to be applicable, and I was looking up the actual help file on the Select-Text-Toolbar in Medium, I wanted to offer another possible explanation. Which is — some people on this site don’t know what the Highlighting function actually does.

https://help.medium.com/hc/en-us/articles/214406358-Highlighting

If you select a section of text and click on the highlighter icon on the toolbar that pops up, a few things happen. One is the author gets a notification that you highlighted it. Another is that if someone who follows you reads that article, they will see your highlights with your name next to them. If enough people highlight that section of text, it will also get a label of “Top Highlight.” And lastly, it will add the snippet to the “Highlighted by” section of your profile — the section you’re talking about. The point of this function is to showcase the thoughts and ideas that you thought were most important in someone else’s articles, and share them with others. It’s a more drilled down version of the Recommend button.

But there are so many self-promotional tools in Medium that I think some people also confuse it as a way to highlight the important passages in your own work. And that’s really not its function.

Also it doesn’t help anything that in that article I linked to above, they use the word “highlight” both for selecting the text with your mouse and using the highlighter icon. Which is how I think people are confusing highlighting text with selecting text. Within Medium, Highlighting means using the highlighter icon after you have selected text. It doesn’t just mean dragging to select. I sent a note to the Help team to suggest changing their wording in that article to make the distinction clearer.