My main is Alliance, but I always level a horde character to see the story from both sides. I normally keep my alts few, but during Legion I leveled all classes to 110 to see the Order hall campaigns, get all artifacts, get all class mounts, and do the mage tower quests. Playing 1 main and 11 alts was sometimes fun and sometimes boring, but thankfully I'm done with them and now focus only on my main until BFA. I'm trying to think now which alts I want to play in BFA.
Are there good reasons to level alts in BFA?