1. Mostly it suits my tastes better. Last weekend I finished a winter supply of plum jam. You can't really buy one that isn't half sugar despite plums being pretty sweet on their own. But yes, it's cheaper, better, and with a podcast on, downright enjoyable. 2. I buy some special ingredients with certain meals in mind but there's base stuff that doesn't spoil easily and I simply keep it around: condiments, eggs, bran…
Home canned brandied diced apples. Tastes as good as it sounds.
Another one you can't buy in any store at any price is home canned peach and rum sauce, which is pretty much pureed peaches and rum.
(edited to note the above two are right out of the Bell canning book, nothing exotic)
I'm well aware neither is very paleo or good for me, but given the effort required to prepare them, its a great special treat.
On a slightly different tangent I find it hard to find fajita chicken meat that is marinated the way I like it, with 1:1:1 ratios of orange, lemon, and lime juices. Why? I don't care if its right or wrong, I just know it tastes really good at those ratios!
The best stir fries I've ever had have been just "stuff from the fridge" all dumped in.