Consider this: when you speak the listeners know what you mean based on the rest of the sentence. When you write you give the reader the intended word through spelling. People who read will see your words and assume you really meant "then" instead of "than", and the sentence will make little sense.
The words "I" and "eye" sound similar, but if you write "eye" I will read a sentence first thinking you are trying to say something about an eye, then when it breaks down, go back and find the issue. End that my friend is less then eye-deal for comprehension.