The words come, cum sound the same but have different meanings and spellings. Why do come, cum sound the same even though they are completely different words?
The answer is simple: come, cum are homophones of the English language.
To advance toward the speaker or toward a specified place; approach: Come to me.
To advance in a specified manner: The children came reluctantly when I insisted.
To make progress; advance: a former drug addict who has come a long way.
To fare: How are things coming today? They're coming fine.
Together with; plus. Often used in combination: our attic-cum-studio.
Vulgar Slang Variant of come.
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.