It's considered disruptive editing.  Basically, it is a thinly veiled insult ("your spelling and grammar suck, and I'm smarter than you"), and often also leads to US vs. UK English fights.  Just not worth it.  A commonly recognized exemption is fixing something that is functionally broken, such as a typo in a link that makes it not work at all. Fixing indent levels so that who is replying to whom is clear, and fixing typos or unclarities in topic headings is cool too.
Anyway, welcome to WP:CUE by the way. -)
—  [talk] [cont]  ‹(-¿-)›