Why do Hindus worship different Gods if they believe in only one God which is parabrahma?
It is because, the one supreme God doesn't offer any material benefits to the worshipper. Rather, because material things cause attachment and bondage, He snatches away whatever material things his true devotee may have:
yasyāhamanugṛhṇāmi hariṣye taddhanaṃ śanaiḥ
tato'dhanaṃ tyajantyasya svajanā duḥkhaduḥkhitam [SB - 10.88.8]
I gradually take away his wealth whomever I show my favour and grace. Then due to poverty his friends and relatives abandon him and thus he suffers distress after distress.
Thus, in the heart of a devotee detachment towards this world grows and it becomes easier for him to think and concentrate on God more.
So people who are desrious of material benefits, instead of worshipping God, worship other demigods, bhutas (air, water, etc.), ghosts, manes, etc:
kāmaistaistairhṛtajñānāḥ prapadyante 'nyadevatāḥ
taṃ taṃ niyamamāsthāya prakṛtyā niyatāḥ svayā [BG - 7.20]
Those whose intelligence has been stolen by material desires surrender unto demigods and follow the particular rules and regulations of worship according to their own natures.
And secondly, people are under the influence of the threefold material nature of satva(goodness), rajas(passion) and tamas (ignorance). Hence, driven by their primary nature they even though believe in one supreme God, worship many other beings:
yajante sāttvikā devānyakṣarakṣāṃsi rājasāḥ
pretānbhūtagaṇāṃścānye yajante tāmasā janāḥ [BG - 17.4]
Men in the mode of goodness worship the demigods; those in the mode of passion worship the demons; and those in the mode of ignorance worship ghosts and spirits.
Only those whose minds are in pure goodness and who do not have any material desires worship the Supreme personality of Godhead.
Now the thing regarding worshipping avatars is that, Parambrahma has no material form. So for embodied beings like us who have a material form, it is very difficult to worship Parambrahma in His all pervading absolute formless form:
kleśo ’dhika-taras teṣām avyaktāsakta-cetasām
avyaktā hi gatir duḥkhaṁ dehavadbhir avāpyate [BG - 12.5]
For those whose minds are attached to the unmanifested, impersonal feature of the Supreme, advancement is very troublesome. To make progress in that discipline is always difﬁcult for those who are embodied.
So instead of worshipping Parambrahma, most people worship His different avatars. But then, as you know, there are followers of Advaita and many other sects who do not worship any avataras but revere and meditate upon the formless Parambrahma.
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