Docker is amazing but not needed. You can compare it to a simpler VM. You can take a docker and run it on any machine. You have an environment that is separate from your host and you and the container can only access it via defined points (volumes and ports).

Imagine you need to run a 2nd Mumble Server. I never set on up but its often that a 2nd instance is not that easy. With docker its easy. The only difference is that you need to use different ports, when you have only one network access or you use a reverse proxy. You can create a 2nd instance to test stuff, without interrupting your productive system. Its a security benefit, because its isolated to some degree and you can remove one easily.

I started using it with MSSQL Server, because I hated how invasive it is on a windows machine, especially I just needed it temporarily to do stuff with it. I'm not a microsoft admin and I know that Servers from Microsoft are a different level. Docker allowed me to start and stop it and remove it very easily. After that I started using it for a lot of and brought my NAS on the next level.

Also one thing worth mentioning are Linux Containerx (LXC). They are in Proxmox but I have less knowledge. It feels more like a full VM than docker but uses less resources. This is the reason why containers in general are more popular. They are less resource hungry than a full VM but have some benefits than running everything on one machine. LXC feels more like a full system, than docker. With docker you rarely get into the system. You may execute some commands, like a create user command or a one time job but don't access it via a shell from the inside (its possible). LXC on the other hand, you use the shell.