People need to understand that Vishnu is the god of sattva guna and embodiment of dharma. This is well established in vedas. So, to teach the principles and establish the cosmic order he assumes multiple forms as per vedas .
- णि पदा वि चक्रमे विष्णुर्गोपा अदाभ्यः ।
- अतो धर्माणि धारयन् ॥
- ato dharmāṇi dhārayan ||
- Viṣṇu, the invincible Guardian, traversed his three strides, thereby upholding the cosmic ordinances.”
Simple definition
- “He who uphods all forms of Dharmas”
The universe itself is sustained by the three strides of Vishnu . Those three strides also represent the cosmic ordinances through which he subdues Asuras and pashus created by Prajapati.
- एको दाधार भुवनानि विश्वा
- eko dādhāra bhuvanāni viśvā
- “Alone, he sustained all the worlds.
Vishnu is the embryo of truth or cosmic dharma /of RTA
- ऋतस्य गर्भं जनुषा पिपर्तन ॥
- ṛtasya garbhaṃ januṣā pipartana
- the ancient one who is by birth the embryo of truth.”
Maitrāyanī samhita
- रविमध्ये स्थितः सोमः सोममध्ये हुताशनः ।
- तेजोमध्ये स्थितं सत्त्वं सत्त्वमध्ये स्थितोऽच्युतः ॥
- “Sattva is situated in the midst of Tejas; and Acyuta is situated in the midst of Sattva.”
- शुद्धः सत्त्वान्तरस्थमचलममृतमच्युतं ध्रुवं विष्णुसंज्ञितम् ।
- “He sees the pure, immovable, immortal, Acyuta, steadfast, called Viṣṇu, abiding within [the] Sattva…”
When gods and asuras fought for share of the universe, Vishnu assumed the form of sacrifice through which gods conquered the worlds which couldn't be conquered. The concept of Vishnu's multiple forms is directly mentioned in Rigveda by Vasishta
- 7.100.6
- किमित्ते विष्णो परिचक्ष्यं भूत्प्र यद्ववक्षे शिपिविष्टो अस्मि ।
- मा वर्पो अस्मदप गूह एतद्यदन्यरूपः समिथे बभूथ ॥
- What is there to explain about you, Viṣṇu, when you declare, ‘I am Śipiviṣṭa’? Do not conceal from us that form of yours, since you assumed another form in battle.”
Taittariya samhita
- विष्णुमुखा वै देवाः छन्दोभिरिमाँल्लोकाननपजय्यमभ्यजयन् ।
- The gods, having Viṣṇu as their mouth/forepart, conquered these worlds, which could not be conquered
Vishnu being the devata of sattva guna is the embodiment of dharma. It's his job to maintain the cosmic order.