what is the importance of silt ?

Silt is the alluvium deposited by the rivers. Rivers collect soil and other fine particles in their coarse through the mountains. This gets deposited on slope of the mountain ,thereby forming plain.

Silt is important because it is the component of loamy soil. Loamy soil is the most fertile soil consisting of mixture of sand, clay, silt and humus. So, silt is important for agricultural purposes.