Although people who have not read the Sherlock Holmes books carefully, or only watched film versions, may dismiss Watson as bumbling and incompetent, that is not actually the case. Watson is a skilled doctor, brave, intelligent, and unswervingly loyal. Thus he works as a foil by suggesting that Holmes is not just smarter than the average person, but that Holmes' finely honed skills make his powers of detection and observation superior to those of someone far more intelligent and observant than average. Watson also has far better people skills than Holmes, and soothes clients and witnesses Holmes offending others by his brusque manner and off mannerisms.
The main difference between Holmes and Watson is that Holmes views everything from the perspective of someone focused on the possibilities of crime. He is particularly suspicious of Spaulding. He also notes the presence of the City and Suburban Bank and realizes that the bank is the most likely target of a criminal, and that the only significance to Watson is the location of his shop, while Holmes focuses more on Spaulding himself.