The purpose of this tool is simple: if you're cleaning or doing other work on your AR-15 and you need to keep the upper and lower apart (common), just put it in your vice (ideally on a magazine mount), drift out the rear takedown pin, tilt the upper forward and insert this tool into the upper & lower holes to hold it in place. Now you can remove the BCG + charging handle and do whatever work you need without the whole thing tilting too far forward. 

You can print this in PLA, or something stronger if you'd like. One note though: it prints most conveniently in a "standing" position as provided, but this is also the weakest given the (admittedly very slight) stress on the pins. As such I'd recommend printing with at least 75% infill. It's such a small part it doesn't really use that much filament anyway.