For me (I use Kavita) it’s because I want to be able to just pick up whatever device is in front of me at the moment and pick up the book where I last left off even if it was on another device
I do the same thing. I’ve tried Kavita and Audiobookshelf and ended up just keeping the books on a network share and then accessing them through Calibre. I am sideloading to a Kindle though.
This is the right answer. I have dockerized Calibre and Calibre-Web for initial intake, then use Calibre-Web's OPDS feed with my Moon+ Android app for reading on my tablet/phone.
Calibre handles type conversions, metadata sync, and file organization.
Calibre-Web works well for browser reading on my PC.
I use Kavita and KavitaEmail to organize and have a frontend for my books, and the latter to email them to my kindle if it's not on there yet. My kavita container is stopped most of the time because I already know what I'm going to read next and just need it up to sync or send new books.
Used to just have my library I exported from Amazon and ebooks com on a single folder on my NAS, kavita helped clean it up a bit.
I also tried audiobookshelf but mostly for audiobooks and podcasts and didnt quite fit my workflow I already had and liked using kavita and Antennapod.
Academics focused, but Zotero indexing a large cloud storage drive.
Let's things organized by subject, tag, author, title, or whatever else I want. Also keeps my notes all in one place. Huge huge proponent and it's open source!
I am using Calibre-Web mostly - but I have run into issues with thumbnail generation after my collection hit around 500000 books. I am just over 600000 now, but a large swathe don't have thumbnails unless I do a manual metadata search. I should probably look for an alternative, but at this point I CBF.
I tried Kavita but it didn't have the features I needed.
I ended up just throwing them on Nextcloud and using Nextcloud sync onto my reader (Box Air 3c)