Because you never know what trivial bit of information may ultimately prove to be vitally important.
Also, the trusted companion (or some other closely related good guy) is often the true, unrecognized hero.The Lord of the Rings -- Sam. He carried the ring when it most needed to be carried. He carried Frodo to the crack of Doom to ensure the ring's destruction. And the overall story ended with him and his happiness, not Frodo or his.The Harry Potter movie series (I haven't read all the books) -- Neville. His actions (killing the snake, the last Horcrux) led to the possibility that Voldemort actually could be destroyed. Plus, he met all the conditions of the prophecy, just like Harry.The Star Wars movie series -- Lando. He destroyed the second Death Star, which (if the ending of the special edition of Return of the Jedi is to be believed) is the event that brought about galactic peace and celebration. Plus -- just look at him! He's the epitome of 1980s cool! Luke, not so much.