DICTIONARIES DON'T RELY ON SPELL CHECK
Cambridge Dictionaries Online
 Since the invention of the computer, people have strayed from the art of spelling. It's so easy to rely on the computer to "Check Spelling", that people rarely use a dictionary--or bother to proof their own work. As a result, papers are often submitted to teachers and manuscripts to editors that obviously have not been read by the writer. The "Spell Check" does a good job of locating words that are legitimately misspelled. However, this function does not locate words that are spelled correctly, but are used inappropriately.
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Computer Webopedia

Free Online Dictionary of Computers 		For example, in the following sentence, the computer's "Check Spelling" function would not locate the misspelling of the word "an": Peter an I went to the park.

Also, many people confuse the words "then" and "than". "Then" is correctly used as follows: "If you do not listen to my instructions, then you will not know how to write your paper." "Than" is used appropriately in this sentence: "I want to stay home, rather than go to the park."

Many technological advances like microwave ovens, cell phones, and computers make life easier. However, this does not mean that it is no longer necessary for the individual to expend any effort to produce a quality product. No one would expect the microwave oven to cook a dinner equal to one that took hours to bake in a conventional oven. No one would expect a cell phone to be as reliable, have the same reception and sound quality, as a conventional "land line". Why would anyone think that a computer is capable of thinking, of finding every human error that was made in the spelling, grammar, style, or formatting of a document?

It is still very important for each writer to read their own work before submission. Poor spelling lowers the quality of the writing and takes away from what may have been an important message. So, if you want to be taken seriously, want to make a good impression, or want to get a good grade, read your work for errors. Oh yes, and when in doubt, LOOK IT UP!!