You could make that argument about any tool Wikipedia editors use. Why should they need spellcheck? They were typing words just fine before.

…except it just makes it easier to spot errors or get little suggestions on how you could reword something, and thus makes the whole process a little smoother.

It’s not strictly necessary, but this could definitely be helpful to people for translation and proofreading. Doesn’t have to be something people are wholly reliant on to still be beneficial to their ability to edit Wikipedia.