Plants can not move around collecting food and water as animals do. So they stock up whenever there is availability. This stocking up is done in the vacuoles. In case of scarcity, the cell uses whatever it has collected in its vacuole. Thus it functions as a storage for reserve food and water. Bigger the storage space, better the chances of survival in unfavourable conditions.
Vacuoles sometimes also function as temporary storage for excretory substances.