‘Upaasana’ is ‘dhyaana’ or concentration or meditation. A form is required for concentration. Reality is always the same and changeless. ‘Brahman’, as the Ultimate Reality, is Formless.
But ‘Upasana’ of a form is done with a purpose, namely, the attainment of a given benefit. The purpose to be attained by worshipping or concentrating on a form differs. The scriptures tell you how to meditate and on what all forms and with what results. For ‘Upasana’, you have to follow the ‘shastras’ or Scriptures.
The different ‘Upasanas’ are all aids in the path to the ultimate goal, namely, understanding ‘reality.’ Scriptures prescribe ‘Upaasana’ in order to train the mind to concentrate. ‘Upasana’ is the affair of the individual; there is nothing collective about it.
Sri Chandrasekharendra Saraswati