As a noob I have a question as to what advantages I have using docker compared to just a service like Heroku where I just push the application to them... and I don't bother with docker?

To me with my limited understanding this seems like just another step.

Now granted when it comes to work, I'm using docker with specifics that I know why I would want / can specify with docker ... but for personal projects this ever comes up for me.

But otherwise for personal projects it just never comes up.

Yet on the other hand when it comes to various examples I see more and more that involve docker where, I'm not sure it needs to / what the advantage is.

Obviously there must be some strategic choices / advantages I'm missing.