On the profile page, one can see year diagram by weeks or months. Now, to see distance/time, one have to click and wait a bit for response (every click sends new request to Strava back-end). The idea is to retrieve these info for all weeks/months right away. And when the user hovers the mouse over the blue bar, distance&time pops up (near the cursor) INSTANTLY. So the user can quickly compare numbers throughout the year.
'Easy to implement, nice to have' kind of feature.
(In training log, it's not so convenient, as you can't see this data compactly within one screen)