The question, why one worships Ishwara, is answered by Krishna (and commentators) in bhagavad geeta 7.16.
मूल श्लोकः
caturvidhā bhajantē māṅ janāḥ sukṛtinō.rjuna.
ārtō jijñāsurarthārthī jñānī ca bharatarṣabha৷৷7.16৷৷
English Translation By Swami Gambirananda
7.16 O Arjuna, foremost of the Bharata dynasty, four classes of people of virtuous deeds adore Me: the afflicted, the seeker of Knowledge, the seeker of wealth and the man of Knowledge.
English Translation Of Sri Shankaracharya's Sanskrit Commentary By Swami Gambirananda
7.16 Again, O Arjuna, foremost of the Bharata dynasty, caturvidhah, four classes; of janah, people; who are eminent among human beings and are pious in actions, and are sukrtinah, of virtuous deeds; bhajante, adore; mam, Me; artah, the afflicted-one who is overcome by sorrow, who is in distress, ['One who, being in distress and seeking to be saved from it, takes refuge (in Me).'] being over-whelmed by thieves, tigers, disease, etc.; jijnasuh, the seeker of Knowledge, who wants to know the reality of the Lord; artharthi, the seeker of wealth; and jnani, the man of Knowledge, [i.e. one who, already having intellectual knowledge, aspires for Liberation.] who knows the reality of Visnu.
English Translation of Ramanuja's Sanskrit Commentary By Swami Adidevananda
7.16 'Men of good deeds,' i.e., those who have meritorious Karmas to their credit, and who resort to Me and worship Me alone - they too are divided into four types according to the degrees of their good deeds, each subseent type being better than the preceding, because of the greatness of their good deeds and gradation in respect of their knowledge. (i) The 'distressed' is one who has lost his position in life and his wealth, and who wishes to regain them (ii) He who 'aspires for wealth' is one who desires for wealth which he has not till then attained. Between them the difference is very little, as both of them seek wealth. (iii) He 'who seeks after knowledge' is one who wishes to realise the real nature of the self (in Its pure state) as an entity different from the Prakrti. He is called 'one who seeks to secure knowledge,' because knowledge alone is the essential nature of the self. (iv) And the 'man of knowledge' is he who knows that, it is the essential nature of the self to find happiness only as the Sesa (subsidiary or liege) of the Lord, as taught in the text beginning with, 'But know that which is other than this (lower nature) to be the higher Prakrti' (7.5). Without stopping with the knowledge of the self as different from the Prakrti, he desires to attain the Lord. He thinks that the Lord alone is the highest aim to reach.
Thus, according to the geeta, some worship BhagavAn for material needs and some worship him for liberation.
