Before writing my explanation let me take the chance to define the aforementioned terms so that those who are new to these words can at least have a ground before reading the explanation further.
Devotion/Bhakti is primarily of two categories, viz. Saakaar form of Bhakti and Nirakaar form of Bhakti.
- Saakaar Bhakti:- This is a phenomenon where a devotee sees/realizes the Supreme being (God) in the form of an idol, picture, or religious symbols (like Shiva Lingam). In this type of Devotion he holds a figure as his beloved deity
- Nirakaar Bhakti:- This is a phenomenon where a devotee is gifted with such a discerning eye that he realizes the Supreme everywhere in physical, or mental plane. These people consider God as formless
Here is the explanation what i posted in that forum to explain the subtle difference between those two forms of Bhakti :
'Saakaar' and 'Nirakaar' - Both paths are correct!
Reason:- It's true that God is beyond the discrimination of Forms or Gender. But unless a person is installed in Bhakti of Saakaar form he can never become firm in Nirakaar form. Imagination is highly important to converge our focus/concentration.
For instance, If i say some random meaningless word, "kambalotaparbahuta" and tell you that he is the Supreme god and we can attain Moksha only by praying to that deity. One may not be able to focus on it since one can't imagine a figure to focus (Imagination is a prerequisite for focus).
Now. If i add a little more spice to it and say "He has a human form but has four hands 2 wings of eagle, holds a Machine Gun (which is his divine weapon/Astra), rides on a dinosour (his Superior Vehicle) and protects the whole world with his grace, and he is very merciful"
Then, with this imagination if i draw that figure, and give it to someone and say worship him daily. Believe me, automatically one will be able to focus and concentrate on that figure for quite some days till one feels that he/she is perfect in his/her Bhakti.
Next, if one is asked to try to meditate upon the same figure in ones inner self (Bhroomadhya Chakra) one will be able to concentrate on ones bhroomadhya chakra properly. And then by and by one would become a master in Nirakaar bhakti.
The sum and substance is, barring extraordinary Saints or 'Kaaran-Janma' humans, normal humans can't focus their bhakti on Nirakaar form so easily (because sense/mind/mann is chanchal/unstable).
Hence Saakaar form of worship is a stepping stone/prerequisite to Nirakar form of Bhakti.