So, you have a Sodastream and have just run out of Sodamix. You could always buy more, but what if you want to make flavors not offered by Sodastream? What if you want to save even more money?
As you may have figured out by now, putting powdered drink mix into carbonated water doesn't work very well (unless you're going for a Diet-Coke-and-Mentos-style reaction). By saving your old Sodamix bottles, you can refill them with dissolved drink mixes and still use the measuring cap to make perfect soda every time.