Shippers will have commodities they need to get moved.  A shipper that grows hops for beer may have a bunch of loads that need to be shipped to numerous brewery locations several times a year.  That shipper will send the when, where and what is being shipped to brokers they work with in what is called a request for proposal (RFP) and ask them to provide their bids for what they would charge.  The shipper then awards the winning bid to a broker that now must find carriers for all the loads they have just taken on.