Starting from no starch
Before you can prove that a leaf made starch today, you have to be certain it had none yesterday.
Destarching
To test that sunlight and carbon dioxide are needed for photosynthesis, a plant that has been kept in the dark for 48 hours must be used. When a plant is kept in the dark for 48 hours, the starch contained in its leaves is completely removed.
Why the starch disappears
Two things are happening at once in the dark. No light means no photosynthesis, so no new starch is made. Meanwhile the plant goes on living — the starch already in the leaves is converted to sucrose and sent away through the phloem to the rest of the plant, exactly as it always is. Production stops and export continues, so after two days the leaves are empty.
The plant is not being harmed, only emptied
Nothing has been taken away from the plant that it will not get back the moment it is put in the light again. Forty-eight hours is long enough to clear the leaves and short enough that the plant is still perfectly healthy — which matters, because a dying plant would not photosynthesise either, and then the experiment would prove nothing.
Where this step is used
The light experiment and the carbon dioxide experiment both begin here. The chlorophyll experiment does not need it, because there the two halves being compared are on the same leaf and have had exactly the same history.
This is what makes the result mean anything
Without destarching, a leaf that turns blue-black might have made that starch this afternoon or last week, and the experiment would have shown nothing at all. Emptying the leaves first is what turns "there is starch here" into "starch was made here, today, under the conditions I set".