It's different for the same reason math teachers want you to "show your work" in addition to simply writing down the answer to a problem. They way you as a human have approached the problem and worked it out is valuable information in both the review of the work itself and in understanding what has gone wrong when something has gone wrong.

Think of your branch as being one long multi-day math problem. If I'm grading your work, I don't want you to show me all the parts you think are neat and tidy and important after you arrive at what you think is the answer. I want to see everything you tried, even the stuff that didn't work.

I'm not opposed to only having merge commits on master, but somewhere, on some branch which is recorded for all of time, I want to be able to see every decision that was made to bring HEAD to what it is right now, on the most granular level possible.