Like any mobile network provider, and any business, Vodafone's primary aim is to make money.
Vodafone is the second largest mobile operator in the world (after China mobile) with 434 million subscribers. In the UK, it has the biggest market share with 19.5 million subscribers. Because of this, Vodafone's primary aim has historically, and continues to be, been to retain their customers by providing excellent service and network coverage.
They have been around a lot longer than some of the newer additions to the market, such as Three, and are therefore viewed as, if not the cheapest, then the most reliable in terms of product and services.
They also have the largest share of business customers, who are less concerned with gimmicky low price offers, and more likely to be retained if they are provided good service.