First, the stones help prevent birds from eating the watermelons. When birds land on the melons with stones on top, they will not be able to stand firmly to eat. Thus, the birds will be scared and fly away.

Second, farmers mark the ripe melons. Watermelons usually take up to a month to ripen after fruiting.

Third, people put stones on the watermelons to avoid sunlight.

Fourth, to let the watermelons grow evenly. When the watermelons grow to a certain size, their fruits will grow vertically. If the stones are heavy enough, the melons will grow more evenly.
Fifth, to improve the quality of the melons