You can buy (and some people do buy) a device that solely allows you to breathe. For instance, a snorkel. Of course, it's more accurate to say that it's a device that allows you to breathe underwater; likewise, it's more accurate to say that a Kindle is a device that allows you to read without carrying around so many heavy books. Similarly, some people pay for cutlery and crockery to allow them to eat (more pleasantly), or telephones to allow them to talk (even when the person they're talking to isn't nearby), even though eating and talking aren't much less fundamental than breathing.