out of curiosity I did an internet search and got more than 1 million hits.
Google shows a brief description .. without link.
The first link ones are PDFs from big semiconductor manufacturers.
I guess they describe it more detailed than we could do this in a forum.
Generally:
* do an internet search and a forum search for general questions.
* start a thread with more detailed questions. .. and refer to the documents you´ve read apon this topic.
Also, here is one in LTspice, the free simulator, so you can run it and see.
Analog.com do some interleaved buck chips, so download one of those and you can read about it.
But in short.
Reduced ripple current in the input and output filters is the main reason.
Also, its the old i^2r law......one buck channel sending 10A through 0.1 ohm means 10 watts conduction loss
wheras two 5A channels each with the same resistance is only 5W overall conduction loss.