The spring exists only to get the plunger to extend after the lifter has bleed down to minimize valve tap on start up. ( engine stopped with valve open )

The check valve isolates cam to valve forces from the oiling system and locks the lifter solid.

Think of a lifter as a floor jack that has a weak pump but good lifting cylinder. If you try to jack up a car, it won't go up. If you raise the jack without a load then place a load on it , it will hold. The added twist for a lifter is the plunger isn't sealed to allow it to bleed down slowly to prevent pumping up too much if the valves float. Another parlance is a timing chin tensioner that has a ratchet.

Also, the plunger area isn't large enough for oil pressure to counteract valve spring pressure very much.