The words berth, birth sound the same but have different meanings and spellings. Why do berth, birth sound the same even though they are completely different words?
The answer is simple: berth, birth are homophones of the English language.
Sufficient space for a ship to maneuver; sea room: kept a clear berth of the reefs.
A space for a ship to dock or anchor: a steamship moored to its berth at the pier.
Employment on a ship: sought an officer's berth in the merchant marine.
A job: a comfortable berth as head of the department.
The emergence and separation of offspring from the body of the mother.
The act or process of bearing young; parturition: the mare's second birth.
The circumstances or conditions relating to this event, as its time or location: an incident that took place before my birth; a Bostonian by birth.
The set of characteristics or circumstances received from one's ancestors; inheritance: strong-willed by birth; acquired their wealth through birth.
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.