Its only use is so that the player can receive a part of the participation reward for the round (the group contribution and coin)

It provides no benefit and does not qualify player for the packs.

Generally speaking, it's not used. But for player who are still on probation (recently left another group to join yours) or simply missed the time for joining the round, this is their only option to get something. Note that cheer can be done until the fight is over, which is after the round start, so late-arrivals c*e it.