Some reasons: immediacy, taste, convenience, quality, lack of price sensitivity.
> Why would anyone buy [prepackaged white bread]
Texture, goes stale slower than fresh baked bread, sizing, taste, convenience.
> Why would anyone buy [brand toothpaste vs store-brand toothpaste]
Taste, familiarity, trust.
> Taste, familiarity, trust.
That's the idea; familiarity and trust can be manufactured through advertising. You can become familiar with a brand name just by being exposed to it, and trust is harder but advertisers have it down to a science in some important ways.
Sure, bottled water tastes a little better than tap water, but I can’t help but cringe at the thought of 600 plastic bottles going into landfill. I avoid bottled water as much as possible, however when required I admit I do purchase 6 packs of 2L bottles for long road trips.
I guess it depends on where you live, but this is definitely not the case for me. And I live in a 111 year old building.
The BBC did a wonderfully subversive 3-part documentary a few years ago called "The foods that make billions", which investigates in some depth the way people's tastes have been shaped to accept anything the food industry comes up with (no matter how disgusting in concept). Well worth tracking down on YouTube or whatever (particularly the episode about yoghurt, which I found grimly hilarious).