It’s what I like also.
It’s been years since I leveled from 1 to max on a Rogue or hunter, so I decided to do it again when the allied races showed up. The heritage armor gives me a reason to level more characters, so I might as well do it on a class I enjoy.
For the dark lady!
as for the original question, I enjoy playing mages, but sometimes i run out of meaningful stuff to do on my main before i run out of desire to play more mage, so I jump on an alt and play them for a bit.
When I noticed what a pain the the butt it is to carry two sets of gear, flasks, potions, food and what not. On top of having two (or even more) of a class it opened up two extra professions for gathering or making stuff per extra character. As for leveling them, nah I got a boost for them I hate leveling. The boost was either with cash after some serious thought or ever since we could purchase tokens from the AH to turn into Battlenet cash I go that route.
edit: Plus this was before Blizzard saved your key binding and spells for you.
I like alts and there are only so many classes so there are going to be a lot of repeats for a sundry reasons.
For example I have paladins of every race because I like the unique mounts or several warriors and death Knights primarily for farming specific plate transmogs.
I have multiple reasons:
- I like levelling and I’m an altoholic, with many more alts than there are classes.
- I want every class at max on both Alliance and Horde.
- I have multiple hunters, running with different themed stables of pets.
- When levelling alts for heritage armour I like to go with my favourite classes.
- A Drust druid is not the same as a dino druid.
- From the perspective of RP, a belf pally is not the same as a dwarf pally. (Admittedly I only RP in my head, but my characters all have their own identities).
- For Plate classes (DK, Paladin, Warrior), it is mainly to have more alts to farm Garrosh for those Tusks.
- For other classes, it’s mainly because I like how certain Transmogs would look on them.
some classes are just fun to level. i dont do it but i see why others would do it.
hides six druids behind her back
I have NO idea what you are talking about sir.
Horde and Alliance are the main reasons.
However, I don’t spec swap. Except for feral and guardian druids. I have never gotten over the spec split there.
I like to build stories behind my characters. So my protection paladin won’t suddenly become a holy paladin.
I love orc warlocks.
I love undead warlocks.
I don’t want to choose so I have both.
I have a few classes I like enough to make more alts for. I have a lot of hunters, I like hunting, taming pets etc, but having just one hunter doesn’t make sense. I like the way different races look and I rarely if ever race change so boom, another hunter. Plus all but one hunter have themes, DiD only tame fire pets, undead only tames diseased or dead, mag’har only wolves or hyenas, nelf only cats, etc etc.
I also have a few monks, both alliance and horde, I like the gameplay. I have other classes too, and really it just comes down to loving to level, quest and adventure and I know which classes I like the most so why not. I never delete high level characters, ever, so might as well level a new one. 
I have multiples of one class on a server since I keep horde toons on a different server than I do my alliance toons. And with having 1 of each class already for class halls and mounts for Legion I don’t have much of a choice if I want the heritage armor.
I have multiple max level druids on different realms for farming. I like to have a healthy amount of gold on each server I play on in case I decide I want to play high/medium/low pop.
Also, I have multiple of the same class on both factions, but I won’t be doing that anymore… Alliance is dead.
This made me laugh. Thank you.
I enjoy leveling and like to do so with one of every spec each expansion so I get a good understanding of how each plays. Jack of all trades - master of none kind of deal lol.
Also I farm for mogs and it’s easier to complete sets when you have 3-4 of each class to run through raids each week. Also alt army for gold & mats. With so many characters just leveling each one can give me a lot of mats for my toons with crafting professions to use.
RP mostly. I like priests. A priest of Zandalar is different than a priest from Silvermoon. A kaldorei priestess of the moon is different than a void elf priest of the dark. So on and so forth.
Those kinds of racial identity things apply to most classes. A druid of Zandalar isn’t just a druid. They’re a dinomancer. A druid of Gilneas isn’t just a druid, but is a harvest witch. A blood elf paladin is a blood knight. A human paladin is a member of the Silver Hand. You get the idea.
It’s useful to have multiple mages due to rapid farming applications. I’ve got this one on Alliance and an orc mage on Horde.
HELL YEAH Brother !! My Banshee Queen not matter what. Kill them all Sylvanas.
Priests are just really unique y’know
imagine if we still had special priest racial traits