To some degree "there can be a ton of different versions of things" only applies to core OS packages. You mention Mac. but what version of python ships with macOS? What if I need a version other than what ships by default?
At a certain point you need to start defining the environment regardless of OS, and docker works as a tool that handles environment definition for literally any program (same thing works for ruby, java, python, etc). It handles more complex environment definition than packages, but is lighter than a VM. It's a middle ground, which is a great compromise for some cases and not for others.
Varying use cases and lots of flexibility is also the reason why linux is never going to just standardize the ecosystem and say "ok, there is only 1 openSSL package now." Some people see the ability to have a version of linux that is completely bonkers in versioning as a strength, akin to how some places have old windows 95 computers they still run because newer versions don't work properly. On linux, you could have old 1995 packages from a specific app, but the rest modern secure packages.
It used to be completely free hosting, that's one thing that was great about it. Same thing made Sourceforge so completely dominant that it took many years for projects to move off it even after more suitable alternatives were made available.
But the main use case was probably convenience. It's a very quick way for Mac and Windows users to get a small Linux VM up and running, and utilize the copious amount of software written for it.
These days it's mostly standard, for better or worse. There are a handful vendor independent ways to distribute software but this works with most cloud vendors. Is it good? Probably not, but few industry standards are.
At a certain point you need to start defining the environment regardless of OS, and docker works as a tool that handles environment definition for literally any program (same thing works for ruby, java, python, etc). It handles more complex environment definition than packages, but is lighter than a VM. It's a middle ground, which is a great compromise for some cases and not for others.
Varying use cases and lots of flexibility is also the reason why linux is never going to just standardize the ecosystem and say "ok, there is only 1 openSSL package now." Some people see the ability to have a version of linux that is completely bonkers in versioning as a strength, akin to how some places have old windows 95 computers they still run because newer versions don't work properly. On linux, you could have old 1995 packages from a specific app, but the rest modern secure packages.