Then, what is the meaning of “Force shipping to the customer billing address”?
Billing address, in general, can be different from the shipping one. Think about a company that has a very specific billing address, but if given an option, could decide to ship an item to a different address.
So if you don’t want users to fill in an additional shipping address besides the billing one, the feature WooCommerce > Settings > Shipping > Shipping Options > Shipping destination > Select - Force shipping to the customer billing address
suggested above is what forces the shipping address to be equal to the billing one. This way a user fills in only one address.
I hope this further clarifies things for you. There is also a support page with more details about this here.
Cheers!