Often explosives are contained within an enclosure on purpose for greater damage to the surrounding area. When they are open by themselves, the damage effect is much less, as the pressure release occurs over a longer time as such. cf a slow kick in teh guts to a fast and hard kick in teh guts...
By containing them, them the gas pressure from the explosion builds up, and when the outer skin / container breaks the damage form the explosive can be a lot more, as the pressure is much higher, and often more shrapnel will be mobilized.
Not enough info, but being in a plastic bag will not always produce a larger and more damaging explosive, depends on the design of the actual explosive package. Used explosives most of career, but not military, but much of the technology we used was transferable from military and vice versa..