Being useful at work is important, because although you are expected to actually be useful some people can pass off looking busy very easily. Actively looking for ways to do more and investing more time and effort to do projects well will impress both your boss as well as those around you. Reaching out to new workers and sharing your knowledge with them will show your leadership. Learning and being the most knowledgeable and skilled in your position others will come to you as the specialist for your job or the tasks involved. All of the above will add to the company, and therefore to your importance at the company.