We mainly buy a Product of a brand which at that moment satisfies our need. The main thing here is the fulfillment of a need. Many a times, we go to a shop looking for a particular product of a particular brand but if the brand is not there we many a times end up buying same product but diff brand e.g. soaps, shampoo etc.
But sometimes if the buying is aspirational like buying mercedes or a BMW you actually shop for a brand.