Question

why do many plants supply sweet nectors to keep bringing their pollenated back to them ? what is the use of it ?

Answer 1

Bees benefit flowering plants by helping the plants reproduce, via pollination. Because plants cannot seek out mates the way animals do, they must rely on outside agents, called vectors, to move their genetic material from one plant to another. Such vectors include bees, certain birds and wind.
Flowering plants carry the male portion of their genetic material in their pollen. When bees fly from one flower to another, pollen is spread from plant to plant. If pollen from one flower is able to reach another flower of the same species, then that plant will be able to form seeds and reproduce.
Bees feed on the nectar and pollen of flowers. Nectar is a sweet liquid substance that flowers produce specifically to attract bees, birds and other animals. Pollen is a powder that contains the male genetic material of flowering plants. Worker bees (bees whose job is to collect food for the colony) land on flowers and drink their nectar. This nectar is stored in a pouch-like internal structure called the crop. In the process of doing this, bees become covered in pollen. The pollen sticks to the bee's hairy legs and body. Some bee species even have sack-like structures on their legs for collecting pollen, called pollen baskets.
Without bees, pollination and reproduction would be practically impossible for some plant species. This makes bees a vital part of every ecosystem they inhabit.