In ShreemadDeviBhagawatam, First Skandha, Fourth Chapter
Conversation between Vishnu and Brahma
शक्तिरूपं महाभाग किं पृच्छसि पुनः पुनः॥ ५२॥
O Highly Fortunate Brahma! even after realizing the Highest Aadyaa Shakti, why are you asking about Her Nature again and again?
यदिच्छः पुरुषो भूत्वा विचरामि महार्णवे।
कच्छपः कोलसिंहश्च वामनश्च युगे युगे॥५६॥
By Her will, I assume this human like body and roam in this great ocean.
And sometimes I assume Tortoise, Human-lion, Dwarf forms in various Yugas.
न कस्यापि प्रियो लोके तिर्यग्योनिषु सम्भवः।
नाभवं स्वेच्छया वामवराहादिषु योनिषु॥५७॥
Who in the world endears birth in wombs of inferior animals? There is none.
O Brahma! do I take these inferior births of tortoise, boar et cetera, because I want to?
विहाय लक्ष्म्या सह संविहारं को याति मत्स्यादिषु हीनयोनिषु।
शय्यां च मुक्त्वा गरुडासनस्थः करोमि युद्धं विपुलं स्वतन्त्रः ॥ ५८ ॥
O Brahma! leaving the pleasures with Lakshmi, who in the world beseeches for birth in heinous and inferior wombs of fish, tortoise, boar et cetera.
Who is that independent man, who leaves the majestic bed of AdiShesha or the grand seat of GarudaRaaja, and becomes assiduous towards the desire of fighting great wars and conflicts?
From the above words of Narayana, it is clear thus- Narayana does not like taking inferior births and incarnations.
So why does He do it?
As in SwiftPushkar's answer, the reason is thus- Vishnu takes Avataara according to need (Human-lion form necessary to kill Hiranyakashipu, Tortoise form necessary to uphold Mandarachala et cetera).
But if above SwiftPushkar's reason is correct, then why does he say- I do not like it. I hate taking birth in lower wombs.
Also, He further declares-
तस्मान्नाहं स्वतन्त्रोऽस्मि शक्त्यधीनोऽस्मि सर्वथा।
तामेव शक्तिं सततं ध्यायामि च निरन्तरम्॥६१॥
Therefore, I am not independent, I am always appurtenant to that AdyaShakti.
I meditate incessantly that Shakti only.
नातः परतरं किञ्चिज्जानामि कमलोद्भव।
O Lotus born Brahma! I do not know anything beyond that Great AadiShakti.
So what is the point in calling Narayana as - ParaBrahma? Though there is no difference in Narayana and AdiShakti, but still