Nirguna and Saguna are the two aspects of experiencing God.
Nirguna is the eternal, all pervading, omnipresent divine consciousness. Nirguna Brahman is pure energy, vibrant and conscious, that is at work in the Universe - The Supreme Parabrahman or the Supreme Self.
Saguna is the manifestation of Brahman in form, any form, which is limited. But the energy behind it is Nirguna.
Why depend on the Saguna aspect? This is because, it is very difficult for the vast majority of people to comprehend the eternal, all pervading, omnipresent divine consciousness / energy. For them it is much easier to have a divine incarnation or figurehead to turn to. In the Saguna aspect the Brahman appears in the human form, as incarnations to lead the vast majority to the Nirguna Parabrahman. The source of this devotion to the Saguna aspect is the Nirguna aspect in the human beings.
Our Self /Atma is Nirguna, about which we are not conscious, and our physical body is Saguna, about which we are too much conscious of. Our Atma is a tiny spark of light encased in the five *Koshas /Sheaths.
Like the river whose ultimate goal is to reunite with the ocean, the ultimate goal of the individual selves is to reunite with the eternal, all pervading divine consciousness. But we all know only like objects /things can merge into one another seamlessly. So as
long as our personal limitations and karmas are not overcome, we cannot reunite with the Divine. Until self realisation occurs we are bound by samsara (the cycles of birth and death).
When each person realizes the Nirguna aspect in him, he attains liberation or Moksha. So for God-realization one has to have Self-realization - one has to become conscious of one's Nirguna aspect.
