Witches are just fun. On top of being a huge pop culture phenomena and tapping into people's obsession with magic, witches typically add just a quirky bit of magical chaos to the game. Casting spells is of course the main appeal, and what those spells do is critical to how fun witches can be. In TS1, witches could do things like water all the plants at once, have magical duels with one another, turn others into frogs, summon hordes of frogs (from which you could try your luck kissing as many as possible to hope for a prince), set fires, put out fires, and a host of other things. They came with a fun, whimsical world where you could go to collect strange plants and animals that could give you the ingredients you needed for your spells and cauldron, as well as a few magical bakery items like cakes and pies. In TS2, they expanded on the theme with similar spells (though no more frog princes!), and this time you could charm or curse others, and almost all spells were classified as good or bad, and you had a morality bar that made you glow with a warm light if you made it very good, or turn green if you went very bad. You also got familiars too, that could be used for gathering ingredients and talking to if you were lonely. TS3 witches kinda stepped back from this, and kept it more in the Harry Potter line of normal looking people with magical abilities. The spells and alchemy weren't as exciting as previously, but still very fun.
Overall, to love witches, you simply must love the concept of adding a little magic and whimsy to your game. Harry Potter, Charmed, Sabrina, et al are all big points of nostalgia for a lot of gamers now, so living out that kind of fantasy is something we want our little Sims to do too. My hope is that witches in TS4 hearkens back to TS1 and TS2, with a mystical world for strange ingredients, and fun ways to get ingredients, interesting pets, and of course alchemy. I also want magical dueling to be a thing, as well as more awe inspiring spells.
