Answer
Since individual graded potentials progress decrementally, they would die out as they spread through the dendrites and cell body, if summation did not occur. Under such circumstances, these unsummated graded potentials would not reach the axon hillock and an action potential could not be generated at the trigger zone of the axon.
Work Step by Step
For summation to take place, the suboptimal stimuli must take place close to one another before the graded potential has decayed too far.
