I am not familiar with this particular recipe but generally speaking, if you are making "sausages" you are making a dough of sorts. If you didn't steam them, you'd just have raw dough. It probably won't hold together and it won't taste great. Steaming them cooks them and binds them. After you steam them, you can grill them or broil them or panfry them or whatever else you want to do to finish them.