Question

treating of fungicides before sowing is necessary ,why

Answer 1

Seeds are treated with fungicides before sowing to avoid seed-borne diseases.
However, seeds may germinate, even if not treated with fungicide solution.

Answer 2

I think the question is "why should seeds be treated with fungicides before sowing?"

Treating the seeds with fungicides is necessary because the saplings or seedlings may be hit by seed borne diseases like anthracnose, alternaria, white mould,etc. The seedlings also maybe affected by soil-borne diseases like wheat mosaic.
To prevent this seeds need to be treated with fungicides before sowing.