Without it you would struggle to put back the bolt as the sear will stand on the way and you will have to strip the whole gun if you want to service the bolt or change the spring. It seems a good idea to simplify the mechanism with one less sear comparing to AS-01, still simulating a two stage trigger, but the actual design seems poor with some redundant bits and not allowing to pull the trigger enough to lower the sear when inserting the bolt back.

Anyway, the fitting is quite easy, see picture 2. The material is plastic, 3D printed.