One of the most important thing docker provides is an abstraction between the application and the OS. If you don't need the abstraction, then it's only going to look like cost.
On the other hand, being able to write an some code in python 3.9 and deploy it anywhere across a heterogeneous production environment without fighting with the OS about how many versions of python it has installed is a useful thing.
And if you're using Java/Node/C#/etc? Where all dependencies are included in the compiled output?
But it ends up being a matter of preference, I could still do it without it, but I'm used to it now, for some reason I feel that it makes my systems more "reproducible" in a simpler way.