Given that brand growth usually depends on increasing penetration, brands need to make sure that the brand/customer experience is positive relative to its price so that trialists are happy to buy the brand again. Hence, activities designed to make the experience more positive/memorable can often be a good investment e.g. loyalty schemes. Schemes whose rewards are highly valuable to the customer but low cost to the brand provide a win-win (e.g. freebies given to customers in order to reduce overstocks). Loyalty schemes that enhance customer value (at relatively low cost) can play an important role in the marketing mix provided the brand still invests plenty of money to attract a much wider audience of category buyers.
Check out this summary of studies indicating that the cost of retaining a customer is relatively low compared to the cost of acquiring one (although the latter is usually required if a brand wants to grow).