(If there is a different, more suitable stack exchange site, let me know.)
I recently visited Bourges cathedral, which does not have a transept, and the interior seemed much cleaner than the usual transepted gothic cathedrals.
Most gothic cathedrals and other big church buildings have a transept, and I always accepted this as the standard way to build one. On the other hand, there are a few without (Bourges, St. Mary in Lübeck).
So, why transepts?
So, it's really not clear to me. Maybe it's just a stylistic convention. (Are there records of architectural discussions about this when designing a church?)