It depends on the context.

Science is our way of observing the patterns in reality and making predictions based on those observations.

If you want to use Marxist theory to test, replicate, and predict/plan for the future then, then yes, it is important.

If you mean "How important is it to our goals of achieving communism" then that's complicated. Scientific communism is very useful for predicting things like how your boss/landlord/ruling class will act. However, if you mean "Is it important when selling communism to others" that's even more complicated, as that depends on who you are talking to. For example, the reason I'm a communist is because it makes sense to me from an ecological standpoint. I see it as the only way our species will survive and thrive and I base that on what I understand about communism and the natural world. So if you're trying to convince someone that has a material world view, it is pretty important.

Hell, half the reason we say capitalism will inevitably fail is because when you make predictions based on the patterns we see in nature, empires, environment, it becauses evident that capitalism cannot continue indefinitely because it quite literally requires more resources than our planet can sustain and it sucks at doing anything other than consuming. It will lead to our extinction. It fails at predicting and meeting the challenges our species faces, and so it will be made obselete or we will die.