In order for subscribers to be able to send and receive SMS text messages globally, the operators to which they are subscribed must connect their networks. This requires complex bilateral agreements between each operator and every other operator with whom they wish to exchange messages. The goal is to achieve complete interoperability, where each network is able to send messages to every other network across the globe.
However, signing a new agreement for each connection would be time consuming and wasteful (especially as every time a new MNO or MVNO comes into existence, they would have to go through the whole process, too). This is where SMS hubs come in. Hubs have agreements with multiple operators to send and receive traffic, meaning that instead of negotiating with all the other global operators, each MNO just needs to contract with a handful of hubs, which will then handle the transfer of messages between networks.