Whats the stat? Is it 8% of all money in the country is spent at tesco? Someone will correct me but its a shockign statistic.
So how do they sell stuff cheaply? By taking advantage of their market share and forcing suppliers prices down to barely sustainable levels. Simple really - the biggest boy in the playground usually wins the fight.
They also make a killing on other aspects od the business - what do you see these days at the checkouts in most supermarkets? Usually masses of adverts for credit cards, bank accounts and every type of insurance you can imagine. Once you're in there you're a captive audience for them to do with you virtually what they wish. Just make sure you don't fall for it and come out on top.