Question

why in most experiments on photosynthesis a destarched plant is used

Answer 1

We test for the presence of starch in leaves in order to determine that photosynthesis has occurred. Glucose - the product of photosynthesis - is rapidly converted into starch for storage in the chloroplast and cytoplasm.
When investigating whether a condition (or a factor) is necessary for photosynthesis is to occur we need to remove all traces of starch in leaves so that we can provide evidence that photosynthesis takes place during the experiment i.e. in the presence of this factor and not in its absence.

Answer 2

It is also the process of eliminating starch reserves in a plant for experiments concerning photosynthesis. This is done by leaving the plant(s) in a dark place for a long period of time. Due to the lack of photosynthesis in this place, stored starch is used up, thus the plant is destarched.