It could be a project with a deadline, and you want to knock some things out earlier so that there is less crunch time needed in a few weeks.
Maybe you want to get some of your work done early so you can take it easier in near future where you anticipate yourself being preoccupied with other responsibilities.
Or, hear me out. You feel secure in your job position, and simply take pride in your work. You will be there working anyways. I would rather get stuff done and feel productive at work than to have meaningless down time twiddling my thumbs, waiting for a response.
Many of us have jobs that are on the whole very good, but where inter-departmentmental or inter-personal challenges come up from time to time. I’m definitely not one who’s willing to quit in search of the elusive perfect company in that case.
In that case, remembering that "it's easier to get forgiveness than ask for permission" is a valuable tool to have in your repertoire.
(Also, not every decision that you need to communicate up to your manager is going to have a significant impact on your company. Sorry?)