It would be helpful to have the option to be able to rename tabs. This could help to easily differentiate between tabs that have the same name by default, such as some email clients or messaging applications.
Renaming a tab would be very useful when you have a website opened multiple times, and each tab is at a different section of that website, because you could conveniently keep track of where each tab is.
e.g. You have some sort of textbook opened, and one tab points to the contents, one tab points to the main content you're focusing on, and another points at a relevant appendix. Renaming tabs would be a great help in this case.
And there are probably many more use cases for this feature that I can't think of.