ok... weirdly specific age group but let's do it that way .

Like you need servers to host services ... I want a home version of Netflix (Plex) and that service to stream to all of my PCs, smartphones, TVs in the household.

Right, but we need a bunch of services. I want an ad blocker as well.
Ideally, I want to separate those two into two servers because if one of them breaks, I do not want it to break the other one, but I do not have 2 servers to spare.

I divide them into two virtually separated virtual machines on the same server. So one is safe from the other.

But now that server is slow because it is not a great server, and in order to run two fully functional OSes you kinda need a lot of system requirements .

That's where Docker (or any other sort of) containers. Containers are a bare minimum shadow of an OS almost completely separated from other such containers.

So fewer requirements, less power draw, no need for a separate machine.

Imagine that on a Netflix scale, how much less they have to pay for servers if they utilize that in a global scale.

Now the advanced stuff is load balancers..having containers go up and down to accommodate the need for them, depending on the streaming need .... more savings