Power
Oh, yeah. Not as much as the judge, but the quality of presentation of your client's case is all on you. If you knock it out of the park, you could save a life. If you fall down on the job, you could potentially be responsible for someone being wrongly imprisoned, executed, or set free when they really shouldn't be.
Much of it will be determined by the evidence, but your powers of persuasion will have a lot to do with it as well. "Good at persuading people" is an excellent special skill to list on your professional résumé.