Intermodal shipping describes the contractual arrangements, rather than shipment mode.
Most international freight door to door shipments are either intermodal or multimodal. Although they sound like a shipment mode, they are both really a combination of other shipping modes.
By using shipping containers, shipments can easily move from one form of transport to another. The mode used for pickup and dropoff is usually road, although rail or barge modes may also be used. The modes used for the main transit is either ocean or air.
Under an intermodal shipping arrangement, the shipper engages different forwarders for some shipment legs. The main reason that this is done is to reduce freight costs, although that often comes with a tradeoff of a longer transit time. The drawback is that it increases the amount of documentation required (including there being several house bills of lading) and the amount of engagement with the shipment.