Typically, it is rare for doctors to fulfill roles other than medical practices for the benefit of those around them, but as the Straw Hats' doctor, Chopper's responsibilities are not limited to just medicine alone.
He occasionally performed tasks like steering the ship as their unofficial helmsman while on the Going Merry, and during the heat of battle, he can also fulfill the role of a physical fighter to support his friends.
However, due in part to his lack of battle experience or formal training (as well as his Rumble Ball's time limited effect) Chopper was originally considered part of the "Weakling Trio" along with Nami and Usopp. This lay largely in his lack of battle intuition and naïveté at the time, though it is shown that he has gotten much more capable after his two years of training on Torino Kingdom.