Question

Why do seeds need wet soil to germinate and grow into a big tree Wikipedia

Answer 1

because the seed needs water and germinate and later swells which in turn hydrates the cell of the seed

Answer 2

It is because food materials in the seed get activated when they absorb water and they get swollen. Water enters through the Hilum and provides necessary nourishment and it also softens the seed coat, so that it can rupture easily and give way to plumule and radicle.
And for plants and trees, water is an important raw material for photosynthesis. So, they need water to grow.

Thanks.