There are a multitude of things but these two are the most common, Mystery and powerful while weak. The mystery comes in with the fact cause they practice magic. Even a world where its all explained there is still a mystery and unpredictable to it that they are able to control. They also come off as exotic cause of the magic they use. The second part comes in is yes, magic is powerful but at the same time physically weak. Being weak is often attractive cause it shows you need to be taken care of. At the same time witches have power which they can use so people who like dom girls like them also. Being able to dominate a magical girl is also another thing, overcoming worldly power and getting to them physically. Men represent physical strength and magic represents femininity. Only natural for the two to come together.
Of course a basic explanation also is that they dress either very little or loosely and that modern times have witches having supernatural beauty, often through magic. Men always want to fuck hard to get and witches represent that also cause of the power they wield. Aside from getting the princess or getting the elf/dryad this has to be the next most common fantasy to a nerd. >>2263358
Go out on a full moon and start a bonfire to attract one. If no fire can be made walking alone in the forest or graveyard is you next best bet but you are more likely to get jumped by a were wolf like that. >>2263282
I used to dream this up a lot. One of them being the "becoming friends with an unknown witch" as you mentioned before but I also like this one: coming across and helping a witch get out of some deadly trouble. Could be anything from medieval times helping her escape from villagers or goblins to modern times where she is being chased by a demon or something and I kill/banish it.