they do it because they are trying to appear bigger than they are. a sort of united force against predators. if they swim about in a ball then they can appear like one large entity rather than loads of tiny ones. also, it confuses predators as its difficult to single out any one fish in order to catch it.
i owe this knowledge 100% to david attenborough - he is a legand
the predator wouldnt be as likely to barge into a ball of fish. they wait until one is seperated from the group so that it is easier to pick out. or they often have certain methods of dispersing them themselves. for example many predator fish will swim below the "ball of fish" forcing them to the surface where they will then be forced to seperate and spread out as there isnt enough water space to keep in their ball, then they are dispersed and it is easy to pick out individual fish.
david attenborough would never lie - it must be true!
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