Retailer promotions are used to attempt the following:
To increase store traffic. Almost any kind of sales promotion will increase the number of people who come into the shop, but retailers would commonly have special seasonal sales.
To increase frequency and amount of purchase. This is probably the commonest use of sales promotions.
To increase store loyalty. Loyalty cards are the main example of this. Using the loyalty card enables the customer to build up points that can be redeemed against products.
To increase own-brand sales. Most large retailers have their own brands, which often have larger profit margins than the equivalent national brands.
To even out busy periods. Seasonal sales are the obvious examples, but some retailers also promote at busy time in order to ensure a larger share of the market.