Not a diehard fan or heavy user of it anymore but it is, frankly, weird you'd even need to ask.

• It's available where ever you have a connection. You can't forget it or not realize you would need it. It's just always available.
• It's off site if something happens at home.
• Assuming you trust them, your data is safe. It's not going to fail. It's managed by someone paid to keep data safe for you. Economies of scale let them have more redundancy and verification than you. No stray atomic partical is going to flip a bit while it is on their servers.
• You can be lazy about it. I keep a working NAS, an older, slower NAS I back up that NAS to, and gave family offsite another NAS that I also back up to. I have many backup tasks and don't rely on cloud storage. However, I need to, at least on occasion, make sure everything is still working... or I can fuck something up if I don't property document or pay attention when doing anything major. Wouldn't need to worry about that if I just used the cloud.

The biggest plus (for me) is "access anywhere". If you've ever needed a document in a hurry, it becomes easier to understand.

I self-host nextcloud on a vps for this and other solutions.

Sharing things with others and off-site backups.

Because it’s easy. If you trust the service and they have backup functionality, you don’t have to do anything, other than install and sign in. All files synced and “backed up”. No need for an external hard drive. No need to remember to backup. Skate files with others easily.

Sure, you can achieve the same by other cheaper means, but then you have to spend the time doing it. And that’s just for the people that knows what they are doing.

Redundant backups. I know that if I throw a file into it, it'll likely survive if my house burns down.

Syncthing is a PITA to set up, especially if you want to exclude certain files from syncing to all devices in your "pool". It's super cool, don't get me wrong, but it's not easy to configure.

For my use case, Seafile is much better. I host it in house and everything I sync goes to the server, then I can choose what I want downloaded to any other device or just grab files as neccessary. I use it backup documents and downloads on each client (Android, Linux desktop, and Macbook). Then if I need a file that I had downloaded to my laptop, but it's not in reach I can use my phone to pull JUST that file easily.

NOTE: Some people do not like Seafile because it stores data on the server in git-like chunks rather than flat files. I have my own way to manage that so I'm not worried. It's also a Chinese owned company. But I don't pay them, I host it at home, and it has not telemetry or internet access

I have my nextcloud instance that back up my important files from phone and computers. I also work with petty heavy audio and video sessions that I need to share and be able to access from other computers. The nextcloud sever makes things so much easier for us. The minute I change something and save it in a audio session, these changes will reflect on other computers accessing the project. That's on top of when I need to share to files to random people, or work on share document in real time with others.I guess not everyone needs these features, but it was a game changer for me so having this services on my own server is actually great.

I think is like a team/corporate solution thing. Idk, i don’t get it either.