When proactive UX design is at its best, the rest of the product team is bought into this work. The product team integrates the users’ needs into important up-front decisions. Those decisions prevent restrictive constraints in the product. Eliminating those constraints reduces new design debt later on and makes delightful design solutions easier.
Without also having proactive UX design efforts, the design team is only fixing problems caused by decisions the product team has already made. These already-made decisions are about what the product will do, how it will work, and what its underlying architecture will be.