– ‘Default’ acts as a multiplicative factor to the vertex goal weights.
– When no Vertex Group is assigned, ‘Default’ sets the Goal weight/strength for all vertices.

– In this example I assigned vertex weights to all vertexes:
– Green zone: vertex weight: 1.0
– Red zone: vertex weight: 0.6
– Yellow zone: vertex weight: 0.0

– As you can see the smaller the Default value, the smaller the final vertexes weights.

– Another example:

Blender Soft Body Strength Default