The most reasonable, likely, and historically accurate answers to this question: 1. Morale breaking: Barbarian versus barbarian (low-organization) armies generally clashed for short periods of time and winning consisted of one side becoming psychologically convinced that they were going to lose, cause a chain route. The routing force loses more casualties in the route than the battle. Who breaks first is almost entirely (typically) based on morale (how the lowest common denominator of troops feel about their chances of success). Once the lowest common denominator of troops become scared, or convinced that they will lose, they run, and then the people next to them run, and then the whole army runs. Berserkers (insofar as they existed) are designed to scare the lowest common denominator; size, ostentatious aesthetics, and lack of fear/humanity. You don't need the whole army to be convinced they will lose, only the lowest morale soldiers, or the ones specifically fighting opposite the berserkers. 164
The troops facing the berzerkers run, or their pre-battle displays convince low denominator soldiers across the whole frontage to lower their morale, and you win. Fear of the men standing next to you deserting you, allowing you to be killed from any direction is the primary factor causing a route; you don't want to be the last one to run.
1. Berzerkers, large warriors with large weapons, with experience, could be useful breaking up shield formations, especially of more organized armies. Barbarian armies do not have the endurance of organized armies (like Romans) and have to achieve victory early or not at all. Having grouped or distributed men who are more effective at dislodging shielded soldiers from shield walls, removing/breaking shields, or causing spacing in the shield wall through extremely violent melee in order to get inside the formation and break it/route it was a key to victory.
Notes: Berzerkers may be over-hyped and there are versions of this in multiple cultures. Ultimately there is a lot we still don't know, or are certain of regarding how groups of men conducted themselves in the killing of other men on the battlefield.
I'm a tattoo artist and part time zoo keeper. Proof reading is for nerds. 358