It depends. If the goal was to exclude women or old people no matter how qualified they are then you're right, they could just be excluded at a later stage. But

It depends. If the goal was to exclude women or old people no matter how qualified they are then you're right, they could just be excluded at a later stage. But if goal was to save money or reach more relevant people based on the assumption that there are fewer old people and women with necessary qualifications then they actually might hire them once they've applied. Now I think about it, even in the first case it could make a difference. Rejecting an application from a real individual based on prejudice might be harder to do, due to a stronger feeling of wrongness, than just unchecking a box labelled "female".