I like the country flags for several reasons.
First of all, when I see an English speaking country, I can safely take the author's production as the gold standard of English. Conversely, for certain other countries, some caution, mixed with leniency, is called for.
Furthermore, the flag gives a hint at what common cultural knowledge I may expect expect from
the writer. Helps in communication.
Last but not least, one can read the post in what he assumes this person's accent might be. Great fun.
I mean, just read my posts wiz a German accent, as I read the Russians' with Russian accent.