In High Performance Liquid Chromatography, why are ratios of solvents used?
Not sure if I can fulfill Ed's "real answer". I like the word you used for tuning solvent polarity- a knob. Modern students may understand this better.

There is no theoretical restriction in chromatography to use multiple solvents or a single solvent. For example in gas chromatography, you always use a pure gas. The reason for using a mixture of solvents in liquid chromatography originates from the concept of "general elution problem of chromatography" which basically says that there is no single column or a single mobile phase which can separate everything. Not only separation scientists use a single mixture composition of mobile phase (isocratic), they can also continuously change the mobile phase composition as a function of time. This is called gradient elution. 

When you do real separations, you have no idea beforehand about the solubility of analytes injected into the column. If these injected solutes are not soluble in a pure solvent they will permanently absorb on the column. A real sample may contain 30-40 compounds. Say a petroleum sample, literally have thousands and thousands of different molecules. Now think yourself: will they all dissolve in 100% hexane?