In any modern IDE "dredging through old commits" means clicking a single button to see who last changed the line. From there it often makes sense to go look at the PR to see a higher level of what was changed. You cannot include all of that context in a single comment.

You can also do that if you think it’s useful.

Going back to the original ticket can offer far more info than what any “why” comment can give. You can see how old it is, if there are any connected tickets, who were involved with it, step by step instructions how to reproduce the bug, etc.