"SaaS" in quotes since it runs in a small production setup with all the bells and whistles (ie. CI/CD pipelines, continuous releases, user signup etc.), but I'm the only user :-)
I'm a wine enthusiast, i.e. not a professional but interested enough to do a WSET2 in my spare time (I'll do a WSET3 when I find time some day). I like to/need to keep track of two things as part of my wine hobby: Wines in my cellar, and tasting notes.
Used to keep the wine registry in excel and notes in Evernote, however especially the excelsheet lacked features, like easy searches from a mobile device, and notes about the wines in my cellar (not tasting notes, as I have plenty bottles I need to taste but havn't yet, and I still need some notes on those to remember where the heck I got them from and why).
Also, WSET2 tasting notes a much quicker to do with the proper template, but copy/pasting text in Evernote became too annoying (again, phone).
So, I build my own app to have exactly the features and mobile friendly GUI I want. I'm the only user on purpose, because then I can keep building and changing features to be just like I want them.
Yes I know there are some "wine tracking apps" out there, like CellarTracker and Vivino, but they dont fit my needs. CellarTracker is closest to my needs but way too clumpsy GUI and not mobile friendly -- I don't have my laptop with me when I'm in the cellar to find a wine for tonight, I have my phone.
Will I every make up the time I spent building it in time saved compared to my excel/evernote setup? Nope, not even close. But it was a fun side project, and I like fiddling with the hosting/Ops part.