Why is the relationship between Sherlock Holmes and Watson so important to the progression of a detective novel?

I think the relationship between these two famous literary
figures is captured through the way that Sherlock Holmes and Dr. Watson actually act as
foils for each other in all of their adventures. A foil is a character that is placed
next to another character for purposes of comparison. Of course, if you have two
characters paired like this, the extremes of each are emphasised and exaggerated. Think
about the relationship of these two characters: what is notable of course if the fierce
intelligence of Sherlock Holmes that Dr. Watson can never attain. When Dr. Watson tries
to use his skills to deduce facts, he is always shown up by Sherlock Holmes. Yet at the
same time, Watson possesses a fierce loyalty to Holmes that Holmes is forced to depend
upon to solve numerous cases. The two characters need each other, but also Watson's
ignorance emphasises the intelligence of Sherlock Holmes.