When partner decides to bid in third or 4th position in bridge…sometimes there bid can be not quite be up to full strength.
So we use an artificial bid of 2♣
So the raise of 1♥/♠ to 2♣ asks opener for their strength
Rebidding 2 of the major shows a hand with sub opening values and any other bid now shows at least normal opening values.
This prevents the partnership from getting to the 3 level when only 2 of a major can make.