Gas though a solvent is full of additives that coat the carb's internals and eventually cause problems. The mechanic in a can cuts and dissolves the varnish.

Some but not all components of gasoline are volatile and simply evaporate away. Those that don't remain as a composite goo we call varnish. It takes a strong solvent to dissolve that varnish and allow it to be washed away.

As long as the Fuel is flowing there is not an Issue ! Just like Water in a Pond without flow , the Impurities Settle out and cause restriction to Normal Flow !

Note: The Solution of Pollution is Dillution...Thus Fuel Additives Bloke !

P.S. Use It or Lose It ! As We Say North of the Border ! :).