I currently have a 128GB 12.9" and a 256GB 9.7" Pro. I use a lot of storage. I don't keep all of my photos on them, but I'll have photos from my last trip away (e.g. 6 week USA/Disney vacation), plus other photos. I'll also have around the next 10 or so episodes from TV shoes that I'm watching. I have lots of apps. I use the Microsoft apps for work, then also have Pages for my own use. I have training videos for work, comics and lots of games, particularly large adventure games. My current usage is suited to the 256GB that I have on the 9.7". I do struggle a bit more with the 12.9" and include less TV shows, say maybe 3-5, plus also less games, training videos, etc.
So with 256GB being suited to my current needs, I've decided to go all in for the 512GB to give myself even more freedom and hopefully use the iPad even more as my main machine at home.
The extra storage can then be used to store more TV shows while travelling, unload photos & video that I take on my iPhone to my iPad while travelling. Plus, now I'll be able to load more family videos in and edit the files to create videos to show people. I haven't done that before, so I'm interested in playing more with iMovie.
I miss the iPhoto app that we used to have on the iPad from Apple. I'd like to be able to do that again on the iPad too.