The words flier, flyer sound the same but have different meanings and spellings. Why do flier, flyer sound the same even though they are completely different words?
The answer is simple: flier, flyer are homophones of the English language.
One, such as an insect or bird, that flies with wings.
The pilot of an aircraft.
A passenger in an aircraft: special fares for business fliers.
A pamphlet or circular for mass distribution.
Variant of flier.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").