Why must pilots remain within assigned altitudes during IFR flight?

Pilots must remain within assigned altitudes during IFR flight primarily to reduce the risks of mid-air collisions. The airspace system is structured to keep aircraft separated both vertically and horizontally. By adhering to assigned altitudes, pilots ensure that they maintain the requisite vertical separation from other aircraft flying in the same airspace. This separation is especially crucial in congested areas, where multiple aircraft may be operating at similar altitudes. Compliance with altitude assignments helps to mitigate the chance of accidents, contributing significantly to the overall safety of flight operations.