Shipping Method Combinations ensure that if a customer orders products that are in separate Shipping Scenarios, the shipping charges are blended together for a single shipping option at checkout. Without creating a Shipping Method Combination, each Scenario's shipping charge will show up separately.

i.e. Vendor A charges $10 for shipping and Vendor B charges $5 for shipping.

With a Shipping Method Combination set to SUM the values, if a customer orders items from Vendor A and Vendor B, they would be charged $15 at checkout.

Without a Shipping Method Combination, if a customer orders items from Vendor A and Vendor B, they would be able to choose between a $5 or $10 shipping option at checkout.