A cordless reciprocating saw is a versatile tool, mostly found used in the construction industry and by DIY enthusiasts. No good tool kit should lack one, as they're able to switch between a variety of taks depending upon what's in front of you.
They're extremely useful when it comes to cutting through drywall and similar, as well as work well on the horizontal. A good quality reciprocal saw will make a neat job of countertop work, the same with flooring, pipework and similar.
Overall, they're the tool to turn to for a myriad of construction related jobs and the better quality ones are worth the extra investment. Much like a hole saw, they have a purpose, deliver great results and will fast become one of your most hardworking power tools.