Question

Why should seeds be treated with fungicides before sowing ?

Answer 1

so that fungus can't attack them while they're developing

Answer 2

Some fungicides applied to seeds prior to sowing can protect seedlings from soil-borne fungi capable of killing seedlings before they emerge from the soil. (L&urn spp.) prior to sowing can improve seedling establishment (Falloon, ... Previous studies considered effects of seed treatments on seedling establishment.
Fungicide seed treatment of ryegrasses to improve seedling ...