Because around 500,000 families will receive income (through work or welfare) that would lift them out of the category considered being in poverty.
There is an argument that the benefit cap should be raised due to inflation and the cost of living. However, some form of cap should remain as an incentive to work. I will add that a benefit cap is not the best incentive to work in my opinion. Boosting self-esteem, confidence, upskilling, making childcare more easily accessible, increasing workplace duties around flexible working etc is a worthwhile investment.