hi,
we put via's between two metals (which has some resistance), we add more via's to decrease the resistance ( resistance to the current flow) and thereby have a free flow of current in these metals.
now imagine, if u have two metals say, metal1 and metal2. and u just have a via between them, the current flows from metal1 to metal2 through just one via which increases the resistance to the current flow.
now if u have many via's , the current can flow between metals in parallel there by reducing the resistance to current flow.
hope this was convincing !