Being copper and aluminium materials with different electrical potential, the contact between the two generates a galvanic cell in presence of an electrolyte (typically humidity), with deterioration of the less noble material (aluminium) and with rapid decay of electric capacitance of the contact surfaces (stack effect ).
The interposition of a bimetallic Cu-AL plate or washers prevent the onset of the problem: since these products are bimetallic (obtained by explosion welding), the copper and aluminium surfaces are molecularly linked, without the presence of an electrolyte that can trigger.