To Franklin, Collinson is a fellow scientist and inventor. They meet in Philadelphia, where Collinson gets Franklin interested in experimenting with glass tubes. They become good friends and Collinson is a strong supporter of Franklin's scientific pursuits. He proves himself to be a loyal friend, a long-term companion, and a stalwart defender of Franklin. Collinson, who's a member of the Royal Society, is the one to first share Franklin's writing with that elite group, and he's also the one who helps Franklin publish that writing. Franklin also turns to him for help later when he needs to meet with Lord Granville.
Collinson functions as a representative of a larger group of legitimate scientists, whose acceptance of Franklin is an important, though small, part of the narrative. Franklin acts laid-back about his science experiments and inventions, but it means a tremendous amount to him to be accepted by the Royal Society and its community of thinkers, of which Collinson is a part.