And as any English major knows, it's an excellent idea to have an actual human being take a look at anything you've written before you turn it in to whatever higher authority (teacher/professor, supervisor, editor), because sometimes another set of eyes will catch things you miss.
This is why every decent writer has beta readers, even if s/he doesn't call them that.
Oh, and on the topic of spell-check functions not catching word choice errors (not my work, I'm not sure who wrote it, otherwise I'd credit the author):
Eye halve a spelling chequer
It came with my pea sea
It plainly marques four my revue
Miss steaks eye kin knot sea.
Eye strike a key and type a word
And weight four it two say
Weather eye am wrong oar write
It shows me strait a weigh.
As soon as a mist ache is maid
It nose bee fore two long
And eye can put the error rite
Its rare lea ever wrong.
Eye have run this poem threw it
I am shore your pleased two no
Its letter perfect awl the weigh
My chequer tolled me sew.