To promote good oxygen exchange in the soil, which leads to a healthy root structure, you need to ensure the growing medium you use to grow your bonsai trees in, is free of fine material which would otherwise clog. When growing medium becomes clogged air is not able to penetrate the root zone, water becomes stagnant and conditions for damaging fungi becomes ideal.
Among the many uses of this sieve set one of them is making top dressing. See the video below.
This soil sieve set includes 3 different screens so you can remove fine material from the basic ingredients you use to make your own bonsai tree growing mediums.