This question already has an answer here:
When hovering over the edited [whenever] ago link on a person's question, could we please show the edit comment in the tooltip rather than the exact time of the edit?
Showing the exact time of the edit is quite useless, especially when you've already told me how long ago it was edited. Showing me the edit comment could be useful if I'm looking at the post and going "Hmmm, what did the last edit change?", or "Damn that question is still ugly and needs some work!".
In summary: the tooltip contents should tell me what, not when.