Synergies in that specific characters set each other up for more powerful attacks, which are essential in taking down as many enemies as possible per turn as the damage you receive will overwhelm you quickly otherwise.

A simple one is using wind magic to turn the enemies facing away from you, allowing a stronger unit to come up and get the guaranteed crit from behind. Or setting up fire walls to funnel enemies into a tankier guy while they get plunked down. An insane one is summoning ice spikes with one mage, then melting it with a fire mage, before using the lightning mage on the puddles to inflict massive damage across a huge area.

Its less needing a specific comp and more needing to know how to maximize what comp you choose, and possibly if they contribute enough to what you are trying to accomplish. A character might be fine enough and crucial to certain setups, but not for your own and thereby not worth using.