I definitely buy for flavour. Also for ingredients, too. I like my jam to be made with actual sugar, rather than glucose fructose syrup, for example. If I go to the expense of buying a jar of pasta sauce, I want it to be roughly equivalent to what I can make myself, with a little more time and patience, rather than being sweet red gloop full of thickeners, emulsifiers and texturisers.
Some supermarket own brand things I prefer. I think Sainsbury's mayo tastes much nicer than Hellman's, which I find rather sickly. Ketchup has to be Heinz, though. It's that thickener thing, again. Sainsburys own brand bread flour gives by far the best results in my breadmaker - the only brand coming close is Doves Farm and that costs significantly more. OTOH, I tried Tesco own brand and the loaf was awful. I do buy DF plain and SR flour.
I do buy own brand butter, but the stuff I get for spreading, rather than cooking, costs as much as the main brands.
And DS2 likes ambrosia rice puds, but we get those from Poundland!