In the unlikely event that an aircraft has engine failure, the aircraft will slow down to min drag speed for the remaining engines.
Consequently, the lift generated will decrease, and the aircraft will "drift down" to a lower altitude where the increase in air density gives sufficient lift.
(Remember lift is proportional to air density and speed^2.)
It is useful to calculate the drift down altitude for an aircraft weight so you know in the event of engine failure, if terrain clearance will be a problem.
Hope this helps.