As nouns the difference between commodity and leech
is that
commodity is (obsolete) convenience; usefulness, suitability while
leech is an aquatic blood-sucking annelid of class hirudinea, especially or
leech can be (archaic) a physician or
leech can be (nautical) the vertical edge of a square sail.
As a verb leech is
to apply a leech medicinally, so that it sucks blood from the patient.