Comments are not always a waste of time, but comments that repeat or tell you what the code is doing (rather than why) are a waste. For legacy code you generally don't have comments anyway and the code is hard to read/understand.

But if you can understand the code enough to write a comment you can likely refactor the code to just make it more readable to start with.

For code that does not change generally does not need to be read much so does not need comments to describe what it is doing. And again, if you understand it enough to write a comment to explain what it is doing you can refactor it to be readable to begin with. Even for mathematical equations I would either expect the reader to be able to read them or link to documentation that describes what it is in much more detail to name the function enough that the reader can look it up to understand the principals behind it.