Close enough; welcome back Mauser C96 chambered in 7.63×25mm Mauser

Strips of stamped metal that hold some number of bullets in a row. Rather than loading bullets by hand, you could use a stripper clip to feed in a bunch of bullets at once into a gun that has a non-removable, internal magazine. Example below.

Maybe the stripper clips are so it can be used in states with high(normal)-capacity magazine bans.

Seems like everything was chosen as a gimmick

Thats just how Kel-Tec operates as a company, they produce wierd designs that are centered around gimmicks like "it folds in half!" and "you have to use stripper clips!" and "its a shotgun with two seperate tubes!". Its really surprising that the company is as sucessful as it is given how niche all of their products are. The internal magazine makes some sense if the goal is to cut down on weight and manufacturing complexity, it requires fewer parts and less material that way. 5.7 was probably chosen for capacity and how Kel-Tec seems to be on a 5.7 kick lately with their pistol that takes P-90 magazines that they released a few years back. I'm pretty sure this is going to be something that people buy to take out to the range a couple times a year, if at all.