The words spier, spire sound the same but have different meanings and spellings. Why do spier, spire sound the same even though they are completely different words?
The answer is simple: spier, spire are homophones of the English language.
One who spies; a spy; a scout.
See speer.
A top part or point that tapers upward; a pinnacle.
A structure or formation, such as a steeple, that tapers to a point at the top.
A slender, tapering part, such as a newly sprouting blade of grass.
To furnish with a spire.
Definitions from The Century Dictionary and Cyclopedia, 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").