What are the benefits of a poster award?
In my field the thinking is


It draws attention to posters from conference participators.
It provides encouragements to individuals (students) to make an extra effort
It encourages discussion about "good poster design"


The selection criteria is always focusing on good communication of good science, never one or the other.  Since such awards are given only to students and there ids usually also a similar student presentation award both selected by a set of valued senior scientists, the award should be and is seen as a merit.