My first ever online purchase was direct from an advert.
I needed 128mb more RAM, so I bought it from crucial. I knew of crucial because a gaming forum I visited had a single banner advert for crucial that never changed, so I always noticed it.
Advertising works, and even if you think it doesn't you're wrong. Every day you'll make purchasing decisions and often you'll tend toward products you recognise the name of. That process doesn't come from magic, it comes from advertising, the majority of which you'll not even notice.
The best-paying sells advertisers make is to their clients and their industry peers.
My purchasing decisions are based mainly on whatever is the cheapest. I do find brands useful, to the extent that if a product is brand-name, it's likely to be more expensive than the no-name variety one shelf below. Saves some mental effort of comparing prices more closely.
The muesli I purchase is store-brand. I have made the decision by comparing nutritional content of a number of varieties.
I have been advertised to to get me to look at the nutrition labels, I suppose.