[acdha]

This is especially important once an organization grows much. Once you start having ops or security teams, different development groups, etc. there's a significant benefit to having one way to manage everything.

A new sysadmin doesn't need to learn that custom way your hand-rolled deployment system handles dependencies, how to see what's supposed to be running on a box, etc. A security person who's wondering whether something is supposed to be listening on a port can at least see that it's something someone went to the trouble of exposing. That QA team or the person who inherited your maintenance coding can avoid learning a new way to ensure they have a clean CI build.

(That doesn't mean that Docker's always the right answer — maybe you've identified an area like networking where there's a reason not to use it — but in most cases the differences are below the threshold where it's worth caring)