This may sound defensive, but its just a genuine answer, as I agree that many photos are pretty pointless.
In my case I uploaded every photo I've taken since I got my first digital camera in 2000, and I do look at some of them.
To try to answer your question directly: I just checked and I have roughly 100 albums with roughly 50 photos, so I expect that I'll go back to those when I want a trip down memory lane, etc.
As for the other ~36k, I don't have any statistics but:
* On ~10 occasions I've sent emails to friends after getting one of the "this day in YYYY" notifications with a link to the photos
* On a few (~4) occasions, I've used the faces grouping to find old, fun photos of friends
* Yesterday I happened to get a "This day in YYYY" notification that reminded me about an event I photographed at my neighbors house, but had forgotten to send the photos (7 years ago, oops)
* I'd rediscovered some DSLR photos I probably would have missed thanks to the "approx location tagging" (that taps in my phones location history)
* I share photos far more frequently since its free, unlimited and auto-upload
* The auto-animations and auto-panoramas have made me go back and rediscover photos from some old trips
* Just the other day my Mom was interested in a photo she remembered me taking on a trip in 2013. I was able to search the place name and find the photo in seconds
I started down the path of Google Photos because my old workflow of manually merging smartphone and DSLR pics on disk and then not having a good centralized album management system felt clunky.
I'm surprised by how often I find myself going to http://photos.google.com now.