It was made very popular by Marcelo Garcia as he used
it against heavier and bigger opponents.
The reason is because the Butterfly Guard keeps your opponent’s weight off of you.
Lot of beginners would be reluctant to play an open guard and would rather play closed guard only.
One must realise that butterfly guard is good for people that have short legs and also it is the base/entry to a lot more guards such as x guard, deep half guard, single leg x guard etc.
Also, it gives you a better mobility than closed guard.
We covered the butterfly guard last week and the following videos have great information on the topic.
Tired of playing closed guard (just waiting for the inevitable guard pass and you are in trouble), start using spider guard.