Goal: Spelling perfection?

Back in the 90’s I went to school for a semester in London. Although I was a student at the University of Maryland I was taught by the faculty of the University of London. It was an interesting semester. I choose London because I thought at least there would not be a language barrier. Little did I know. My papers regularly came back with my spelling corrected, for example I spelled ” color” and my professor corrected it “colour” by the end of the semester I was irritated but amused. I will never forget my experience. I would often think to myself during that semester what was the point of those corrections. Was that professor trying to teach me the ‘correct” way of spelling the English way. Why did he waste his time on spelling instead of addressing the content of my paper. I never did discover the answer. I did not have a language deficit, but how important is spelling anyway?