An easy reason to self host apps: share media with the household. Sure you can have a samba share and have people browse trough a folder to find the good movie. Or they can load jellyfin or Plex and have a netflix like interface that remembers where you left off and all the fun stuff. You can then add other stuff that will automatically search, download and organize new TV shows episodes for your, etc...

Another reason is some stuff are only possible when hosting. For instance, grocy is an app that lives on a server and manages your stock of food. You can scan goods with your phone and add them to your stock. You can scan them when you throw the can/box etc to say you used it up.

Then it generates a shopping list for you. The nice thing is that it lives on your server, not on your phone. So if more than 1 people do the shopping, you can have a synchronized shopping list, and update it in real time. And the self in self host part is cool because you decide who sees this and no Google or Amazon makes a profile out of your shopping habits.

You can have an online office suite that works in your browser without anyone unauthorized seeing your files.

You can have a bookstack wiki, where you put notes about the house, or whatever you want, and gave it being reachable only by you and people you allow, without a lot of account management.

You can have your own nextcloud, so you have file sync, calendar, etc, without it going at Google or apple. And it is on your server so you can have as much or as little data backup as you want. And often a good fiber line is cheaper than a VPS or a full dedicated server...

With all of this, you can seamlessly switch between multiple computers . You can also manage the loss/destruction of your laptop. Or phone. You can have a local equivalent to Google photos with photoprism.

You can have a frigate server for video surveillance and object recognition, but all in local, your video files don't leave your house. A s it will do local AI stuff.

Last but not least: when you self host your stuff, you can still do a lot when the internet is down. You can replicate services on your laptop if you want. You do whatever you want.