why do we find it useful to have two units for the amount of substance , the kilogram and mole ?
Kilogram and mole measure different things. Kilogram is used to measure the mass, the mole is closely related to the number of particles. 1 mole of a substance always has (number of Avogadro) particles. Therefore, you can't say in general how many moles are in a kilogram, this depends on the substance. For example, a mole of hydrogen atoms has a mass of 1 gram, while a mole of oxygen atoms has a mass of 16 grams.