When the sun goes down at the end of the day, its the light of the 'deep' or 'diya' or earthen lamp that dispels the darkness. Darkness is associated with sadness, lifelessness, ignorance etc. Light or 'Prakash' is Lakshmi, who gives us the wealth of brightness.
Deep Lakshmi brings joy, celebration, 'gyan' or knowledge and life. She shows us the way even in dismal, dull, and dark situations. Deep Lakshmi signifies the 'ray of light' that inspires us towards life, ideas, and progress.