​​​​Why Plants bent toward Light?

The higher concentration of Auxin on the shady side causes the Plant Cells on that side to grow more so it bends toward the Light. Because light is the Energy source for plant growth, Plant shave evolved highly sensitive mechanisms for perceiving light.
It is called Phototropism. Not all plants bend towards light. Monstera and Philodendron, are instead negatively Phototropic and direct their Stems towards the shadow cast by Tree trunks, which these vines need for support.