In Multi crew its vitally important that each pilot knows his/her role and duties.
Briefings are not just about roles and duties (which can be found in the ops manual) they are an opportunity for PF to mentally rehearse what he is going to do and in what order during and after a particular event, at a particular airfield, involving the departure, approach, landing and taxing. It also gives an opportunity to reduce the startle effect of an emergency. This applies to two or one crew but obviously needs to be simplified with the single crew lower level VFR operation and quite a few items are better briefed before getting into the aircraft
The major difference with a multi crew briefing is that you are also inviting PNF to take part with an opportunity to challenge or add to the briefing and this is vital to the safety of the operation, as well as being good CRM especially with a difficult approach or departure.