Seeds are acquired when foraging from plants. Seeds contain the DNA of the plant, and since every world have their own plant species it can be nice save seeds of nice looking plants. But apart from the decorative uses, seeds are normally considered a useless byproduct of foraging and can be tossed away.
Seeds can be planted in flower pots, showing the plant and allowing harvesting of the plant's product.
A more advanced empire can fill seeds into a genesis device and detonate it on a world, changing all plants on the world to those plants.