why do they need to summon Bam

Bamm left the FUG team so he is out of the game they are playing. Khun and team need Bamm because FUG wants to kill them all and Bamm is the only one strong enough to stop FUG members. The only way for Bamm to re-enter the game is by getting summoned by a player.

Like Beta for example in this episode. He is already on the test field, but can’t enter the game cause he is not in any playing teams. He can only enter when FUG member gave him a parasite player from Snake Lady team.