There are many instances where one side may have some commodity that thier opponents would like to aquire. The cargo maybe something that will benefit whomever controls it. Often aquesition may simply be damaging to the enemy and it becomes a target of opportunity for thier opponents. The cargo by definition is mobile to some degree and it is this fact along with its value are what usually bring about these plots. One side wishes to relocate the cargo to a more secure place or to a location where it is badly needed. Clever or desperate opponents won't miss the opportunity to capture or destroy such important targets.

1-10    Supplies must reach the defenders of a besieged outpost or they will
           surely fall.  A small force that won’t attract too much attention is
           needed to try and bring much needed supplies through enemy lines
           to the outpost.