The purpose of boiling a leaf before testing it for starch is to stop any more reactions from occurring in the leaf. If the processes inside the leaf are not stopped, then the starch could be used and not be picked up in the next steps of the test.
Starch is stored inside of starch grains in the leaves. These molecules are too large to escape the storage area of the leaf, so they are always there for the plant to tap into for energy reserves when needed. The starch molecules are made from molecules of glucose strung together to make the larger ones. When a leaf gets ready to use the starch, it breaks it back down into glucose molecules.