Silly, Bonuslink works by selling info to market research company.

They have all your details. Your name, your phone number, your address, your credit card bank, Where you stay, what is your job... etc

Not only that, they know when you spend, where you spend, what you buy, how much you spend. What time of the year you spend most. What time of the year you spend least. As for petrol also (Shell), they know how frequent you pump, which outlets you pump.

Then they sell this statistics to research companies so other companies can make use of the data. For eg. When to have sales. Which outlet to sell more expensive. Which outlet must sell cheap. You will be surprised, one of the highest earning industry is the info and data providing industry.