Jacques is the harbor of negativity, the one to deplete the joy of life, the black hole, pessimistic, and depressive man whom Orlando and Rosalind detest so much, as well as everyone else who comes across him.
His role in the story is interesting because his behavior is often what is expected of life: To hope for the best by expecting the worst, to never believe in fantasies, and so on.
However it is his behavior what reinforces the positive and fantastic behavior of everyone around him. In the end he is an isolated soul who tries to bring everyone with him in his quest to suck the life out of everything, yet, it is this exact sort of thing what makes the characters around him even stronger. This is why when we read the comedy, we can feel the strength of the good emotions to be more evident than the negativity embodied in Jacques.