O Kaunteya, even those devotees who worship other deities with śraddhā (assiduousness), are also worshipping Me (the Supreme) alone, though improperly.
Worship is a process to get connected with whoever is worshipped. This linking has to be in one’s own inner being, the mind. Any external effort is at best a help to involve the mind. Hence Krishna said that exclusive application of mind and thoughts is the essence of devotion.
Why then so many elaborate ways of worship, using flowers, milk, ghee, honey, and so many ceremonial arrangements and symbols? It is only to collect and fix the mind on the act of worship. Devotees find it hard to access and apply the inner mind without these aids.
God being omnipresent, none of these is necessary for real worship. For, whatever is done as worship, actually acts on the devotee himself, just as decorations done to the face reflected in the mirror fall on one’s own face. Thus a variety of elaborate practices came into vogue. Krishna does not wish to deride them, but wants to point the truth about them.