Customers buy a particular product because of the brand name. Secondly the service or the quality of the product provided by the company. Thirdly , it is the tangibility and the reliability of the product assured by the company. Also a very important factor is the price offered by the company and it's competitors. Hence all these factors together contribute towards the buying of a product by an individual. Hence any organisation which wants to reach the level of success should always keep these factors into a consideration.

Okay, to begin with let's look at ourselves and try and answer this question. we all are customers to a plethora of products available all across the world.
Let us take an example of the refrigerator we had last bought and ask ourselves, why did we buy that particular refrigerator when there were lots of other choices available.
1. Price: price plays an important part in picking a particular product and taking it home. Most of the buyers have a particular price range in mind when they go shopping for electrical appliances.
2. appearance: Aesthetics come next. Most of us want our refrigerators to be not placed in the kitchen because the fumes from the cooking might make the surface of the refrigerator oily, hence refrigerators find their place mostly in the living room. Thus making appearance an important parameter in choosing them.
3. Functional proficiency: this is absolutely important a parameter to consider. Without functional proficiency a refrigerator is of very little use.
4. points of difference: next comes the value addition, the particular brand has to offer. Some refrigerators come with a cooler, while there are others that come with a triple door technology, some are power savers. Different brands have different things to offer.
i think by now, we do have an idea as to why customers buy a particular product.

This discussion is more relevant to the brick and mortar format, intangible products and service industry. In case of e-commerce there are some other verticals which impact the buying decision.
An established brand attracts customer because of its long presence in the market. However to retain customer for a long time the salesperson play a major role. These front-line employees are the extension of the brand and they create impact in the customer’s mind and the sales conversion is done. The effort of the salesperson increases manifold in case of start-ups. If you are going to make an entry in the market, your salesperson is your primary weapon to establish your brand. After they convince the customers to try your product then only your service or product will get a chance to prove itself. Let us take an example. You want to enter into the life insurance sector, where LIC holds more than 70% of the market and another 22 companies are present already. So what will be your strategy to gain some share in this market? According to me, you need to hire extremely smart sales people after developing some good products.