When we issue a rain check on an item, we promise the buyer that the item will be available at a later date at the same price. Some companies use rain check by selling the item at a lower cost. In such cases, when the retailer runs out of stock, the rain check serves to bring the consumer for a second visit.
The whole purpose of rain checks is to give customers a second chance who had a trusted and tried to purchase the product during the sale event. This also helps the company to encourage the customers to return again.
This term has originated from baseball match. A game was postponed because of rain and in order to please the spectators, they were given a check that could be used to attend a future game.