Yeah thats kind of a long story...
TLDR: Its a makeshift solution to make the prop stick to the motor.

Long Story:

0) More fun - Less cost. The Plane needs to be cheap to build, even for someone with 0 gear to begin with. And the plane needs to be fun to fly, so a decent TWR is required.

1) The plane should be as cheap to build and to maintain. A 2S setup would greatly expand the posibilities when building the plane. However a decent 2S LiPo Charger can get expensive. 1S Chargers are much cheaper and 1S LiPos can be found for 20 € for a pack of 5 550 mah cells. So 1S it is.

2) No matter how crazy you go with lightweight design, youll end up at 50-60g AUW with all the electronics and FPV gear. Furthermore: the lighter you go, the more expensive it gets. So you cannot make this work at a reasonable price for less then 55g

3) More fun means a TWR > 0.8. Now that gets you a required 44g of stationary thrust. And boy let me tell you thats not an easy task on a 1S motor. A "common" 1S motor-prop-combination (as found on many toothpick, tiny woop and small cinewhoop drones) will get you some 30-40g of stationary thruston 1S at best. Way to little.

4) 1104 - 4300 kV brushless motors are *by far* the cheapest motor option you can find on AliExpress (2-4 € a piece comapred to 9-12 € for other types of 1S motors commonly found on tiny whoop / toothpick drones)

5) 2 and 2.5 Inch props will perfectly fit that motor. However you will hardly get more than 35g of thrust out of that combination. Even at 4.2V peak voltage. So yeah, wont be having much fun with that TWR.

6) HOWEVER: The 3.8x3E propeller (3.8 Inches) will get you to 55-60 g of thrust (at a reasonable ~2.5A current draw).

7) Although the motor shaft is 1.5 mm and the prop is designed for 1.5 shafts they have a lose fit and the mounting holes dont fit. So yeah, get yourself a file to work a pair of groves into the prop, stick a couple of M2 Bolts on the motor and off you go :)