At most grocery stores, it's only available in 1L cartons - and sometimes you find it, sometimes you don't, sometimes it has a long expiration date, sometimes a week or less! It can become quite a nuisance when you bake to order and you suddenly get a last minute request but you are out of buttermilk, or you buy more than you need and end up tossing out half the container because it goes past due. Yes you can freeze buttermilk - best to do it in small amounts which can thaw easily, but again that's a nuisance.
Recently we brought in dry buttermilk powder - all the essence and qualities of buttermilk but in dry form just waiting to be reconstituted. Now I have buttermilk on hand whenever I need it - I can make a little or a lot depending upon my needs - no waste! And this is true buttermilk like the product from days gone by - it is the residue left over from making sweet cream butter, and then dried into powdered form. Today's commercial buttermilk is cultured, i.e.: it is milk with an addition to it to create a buttermilk taste & texture. For more info about buttermilk, see this Wikipedia post.
So what can you use buttermilk for? It is excellent in baked goods; from muffins, to quick breads, to cakes - it gives an amazing texture & flavour to your baked items. Here's a quick non-cake related recipe in which buttermilk is the star - Irish Soda Bread. This bread is VERY easy to make, takes less than 5 minutes to prepare, can be on the table in 45 minutes, tastes amazing and has 4 simple ingredients! It's homemade and your family will think you hit a home run! Try it!