Question

Why do we need to destarch the leaf before performing the experiment?

Answer 1

Destarching is not necessary unless your experimental approach is to measure starch formation.

But if you wish to detect the starch formed by photosynthesis in the light, then it is a good idea to destarch the leaf by keeping it in the dark at least overnight before the experiment. Otherwise you may have too much starch left in both your illuminated leaf and in your non-illuminated control leaf, and may not find any difference.

For very similar reasons, it may also be a good idea to remove chlorophyll after the illumination treatment (always do the same with the control, too) to make the iodine-colored starch better visible.