Fuel needs to be finely atomised for it to burn properly, and an altered spray pattern can result in fuel droplets that are too large to be properly burnt. Although generally passing harmlessly through the exhaust system, the unburnt fuel is clearly not producing power and the inefficient combustion ultimately means the engine uses more fuel to produce a given power output as a result.
Although less common than fuel injection these days, carburetted engines also experience similar problems from deposits because carburettors also use a number of small channels and orifices to meter and vaporise fuel.