For me the biggest is probably Jellyfin. Before, I needed to use external drives plugged into my TV, then browse them using the TVs file browser. I didn't see which movies I already watched, or at which episode I stopped. When I wanted to watch something on my computer, I had to get the drive and plug it in there. The same for when I wanted something new.
Now, I have Jellyfin running on my server, all the clients have access to it and I can watch my stuff whenever and wherever I like. It's also easier to share something.
For me the biggest is probably Jellyfin. Before, I needed to use external drives plugged into my TV, then browse them using the TVs file browser. I didn't see which movies I already watched, or at which episode I stopped. When I wanted to watch something on my computer, I had to get the drive and plug it in there. The same for when I wanted something new. Now, I have Jellyfin running on my server, all the clients have access to it and I can watch my stuff whenever and wherever I like. It's also easier to share something.