Prathy Kamasani gives us several good uses of the Power BI visual header tooltip:
When we look at data journalism posts, most of the times they have annotations, explaining what visual showing or talking about measures. Again most of these data storeys are used for paper. But in the digital world, we do see these annotations more interactively. It is nice to have this kind of lil annotations for everyday reporting as well, and Tooltip Icon can be used for that purpose. Another thing is using canvas space wisely, it is important, and having this kind of hint helps us on saving the canvas space.
Click through for instructions on how to enable this as well as smart ways to use them.