The words grate, great sound the same but have different meanings and spellings. Why do grate, great sound the same even though they are completely different words?
The answer is simple: grate, great are homophones of the English language.
To reduce to fragments, shreds, or powder by rubbing against an abrasive surface.
To cause to make a harsh grinding or rasping sound through friction: grated her teeth in anger.
To irritate or annoy persistently.
Archaic To rub or wear away.
Informal Enthusiastic: a great lover of music.
Very large in size.
Larger in size than others of the same kind.
Large in quantity or number: A great throng awaited us. See Synonyms at large.
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.