Question

why ploughing and labelling of soil is done before sowing seeds

Answer 1

ploughing and levelling of soil is done before sowing seeds to breakdown the large particles of soil and also to make the lowest soil come up and upper soil to go down

Answer 2

Ploughing is done in order to make the soil looser so that the roots of the germinated seed will find it easier to penetrate through the soil.
Levelling is done so that the plants will be able to get equal amount of water.