There are various ways to build a reaguard. Vanguards usually only have 1 purpose, you deadly nukers on standby.
Rearguards can be built for
Hybrid Combo building.
Rearguard with powerful cards can boost a lot per 10 mp, but cannot affford hitting enemy too many times before knocking them out.
Weak rearguard can boost and attack to keep the combo going, but their boosts are weak due to using weak cards... but the cost to build attack combo is cheap as the cards are usually 4 MP.
Rearguards have a lot of concentration, vanguards can go afk until the last 10 minutes of battle after their rear has setup the field for the vanguards to shine.
Van and rear are both important, rear fights throughout the match, Van is the nukers that ensure victory and point domination.