In reply to saikanthan7798:
For one, value conflict resolution. If all signals were the same strength and you had two wires connected together (via assigns or port connections) that were each driving a different value, the resolution would be unknown (X).
If one of those wires is specified to be at a higher signal strength, it will win and the resulting output will take on that stronger value.
This is commonly done to model pull-ups or pull-downs. I haven't seen much usage beyond that except for the occasional use of "supply" strength signals for power/ground pins.