In Judo a failed throw will end up with one or both Judoka transitioning to the ground, where the encounter can be won or lost by hold, choking or arm-lock.
In all martial arts with a Ne-Waza element (such as Judo or Brazilian Jui Jitsu) good initial control over an opponent is required before a submission technique can be applied.
The 'turtle' position (all-fours) is an effective way to defend from attack on the ground. In Judo competition both players are stood back up on their feet if no progress is made quickly after going to ground, and the turtle position is effectively used by many Judoka to stall any kind of progress.
Because of this a large number of techniques have been developed in Judo to open up or to turnover an opponent in a turtle position.