Years ago I bought a Canon EOS-3 (which I still have and use) and Canon had a special offer on at the time, I could have a free Speedlite or a free power booster grip (battery grip), which took 8 AA size batteries, so I chose the battery grip on the grounds that I could buy a 3rd party flash at about half the price of a Canon speedlite but couldn't (in those days) buy a 3rd party PB-E2 battery grip! Also, if I couldn't find one of the 2CR5 lithium batteries the camera used, I could run it off AA batteries using the grip. Plus, it also increased the FPS rate to 7 frames per second. I staked my claim and the grip arrived and fitted the camera like a glove and looked great, it also doubled up the shutter button, function wheel, etc. when using the camera in portrait position, and build-quality wise it was a thing of beauty.
Long story short, other than testing it to make sure it worked I've never used it, as it made a pro-spec weight SLR even heavier. However, I've kept it and I'm still glad I chose it. If 2CR5 batteries ever become obsolete (or very expensive) I'll still be able to use the camera with the power grip fitted and loaded with AA batteries. If I ever need 7 FPS from a film camera (unlikely) I can use the grip. Also, if I'd chosen a free Speedlite it would probably be worth about £50 these days if I could find a buyer, whereas the PB-E2 grip seems to be quite sought after and changes hands for over £100 these days in mint condition.
I've also bought a battery grip for a Canon EOS 30 film camera that I use; once again, if the lithium batteries the camera takes become obsolete or expensive I can run it off standard or rechargeable AAs. Would I buy a battery grip for a modern DSLR? Only if it made a small camera easier to handle... but then again, I probably wouldn't buy a camera that didn't fit my rather large hands in the first place. Or if I was going to keep the camera for may years and the battery grip gave a degree of future-proofing against not being able to replace the battery the camera used if it stopped holding its charge and I could run it via standard AA batteries via the grip.