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 also use SponsorBlock for YouTube, which skips sponsor segments in YouTube videos (and optionally other kinds of segments like intros, self promos, etc.) it’s crowd-sourced for identifying the segments but for almost all the videos I watch someone has already marked at least the sponsor segments

Tree style tabs, which gives vertical tabs that you can arrange in a hierarchy to keep related ones together

Simple tab groups, which lets you have multiple sets of open tabs you can switch between (can you tell I have a problem with too many tabs?)

Unstick!, which when clicked removes any sticky elements, i.e. parts of the page that stay on your screen while you scroll. It’s great for removing all the bars and obstructions to reading that pages like to put in your way. For some reason I have to click it twice for it to work

Read aloud, a good text to speech extension to read pages or parts of pages to you. It can be used with cloud based neural voices from Google and Amazon with some setup

Consent-o-matic, which gets rid of the cookie consent popups for you and it’s configurable as to which types of cookies it will refuse or consent to for you

SponsorBlock for YouTube, which can auto skip sponsor reads and various other kinds of segments you select to be skipped

A few short months ago I would have said RES but, well 🤷‍♀️

One thing to look at if you’re going this route is whether your router supports NAT loopback (a.k.a. NAT reflection or NAT hairpinning). This feature means that you can access your server via the external IP (and therefore via the ddns domain name) even from within your network. It’s really useful for phones and laptops that might be on your home network at some times and off somewhere else at other times, so you don’t have to change configurations on e.g. the Nextcloud client, or remember to type in different addresses inside and outside the network. Some routers just do this, some don’t, some it’s a setting you have to turn on. The router built into my ISP-supplied cable modem didn’t support it so I got my own router and put the ISP one into bridge mode.