Starch is insoluble, so doesn't affect osmosis. Also it is more compact, so easier to store large amounts. The starch is used by cells as a source of glucose for respiration when it is dark.
Why do plants convert some glucose to amino acids?
Amino acids are the building blocks of protein (for growth and repair). Plants need amino acids for protein synthesis. (Plants are responsible for producing many of the amino acids that we rely on too)