For plant growth, sunlight plays important role. So if seed is present under the soil, then how it is growing without sunlight?

A seed usually contains plenty of sugars which it uses up first. A plant has mitochontria-like structures in its nuclei, which can use sugars to generate ATP.

Plant contains even more sugar than its seeds have, then why plant need sunlight if they have more sugars in it?

It's a question of energy. Seeds are constructed in such a way by the parent plant that they can generally provide their own energy through the sprouting process. Then, at some later point this energy from sugars inside the seed becomes exhausted and must come from another source. That is where the sun or some other chemical process comes in to further nourish the plant as it continues along its growth path.

Plant contains even more sugar than its seeds have, then why plant need sunlight if they have more sugars in it?

The first leaf-like structures a seedling produces are called cotyledons and these hold the initial energy store until proper leaves are formed and can photosynthesise new energy reserves.

Plant contains even more sugar than its seeds have, then why plant need sunlight if they have more sugars in it?

 

The sugar in the plant has been synthesized using sunlight as a source of energy. (Ultimately, that is where the sugars in the seed came from as well.)