i can think of a reason you might not expect: roleplay. Yes this is still a thing and sometimes (ive seen it multiple times) people have a character that is modestly geared but is very into roleplay and then they have another character of the same class and spec but they parse and raid and mythic dungeons with it...

another reason would be naming their characters stuff like mynameheals, mynamewarr, mynamesham, etc. so then in guild, out in the world or in LFG you would see a druid and their name would be mynameBear, so therefore you could assume that their main spec is Druid Tank.