A sausage stuffer is definitely worth the investment. It will stuff a more consistent product and can be easier. Stuffing off a grinder also has a tendency to overwork the meat and cause smearing (or loss of particle definition).
That being said, you absolutely still can stuff off a grinder, but it’s just not the ideal method to produce the best final product. Starting with just a grinder is a great way to just get started and begin sausage making, and as you advance, then start finding new equipment and tools to start adding to your collection.