Why a specific Name of Shri Vishnu is used for a specific task ?

Vishṇusahasranam is a famous stotra (A hymn of praise) of Shri Vishṇu. Amidst recitation of these thousand Names, Shri Vishṇu is ritualistically worshipped with an offering of tulsi (Holy basil) leaves or lotus flowers. Although, all the Names mentioned in Vishṇusahasranam are Shri Vishṇu’s Names, according to the doctrine ‘Word, touch, form, taste, fragrance and their energies coexist’, different Names activate different Shakti (Divine Energies). This can be understood from the lines of the following stotra.

औषधे चिन्तयेद्विष्णुं भोजने च जनार्दनम् ।
शयने पद्मनाभं च विवाहे च प्रजापतिम् ||१||
युद्धे चक्रधरं देवं प्रवासे च त्रिविक्रमम् ।
नारायणं तनुत्यागे श्रीधरं प्रियसंगमे ||२||
दु:स्वप्ने स्मर गाेविन्दं संकटे मधुसूदनम् ।
कानने नारसिंहं च पावके जलशायिनम् ||३||
जलमध्ये वराहं च पर्वते रघुनन्दनम् ।
गमने वामानं चैव सर्वकार्येषु माधवम् ||४||

Meaning : Contemplate on Shri Vishṇu while taking medicines, Janardan during meals, Padmanabha while sleeping, Prajapati during a marriage ceremony, Chakradhar (Shri Vishṇu / Shrikrushṇa) during war, Trivikram during a journey, Narayaṇ at the time of death, Shridhar during intercourse with the spouse, Govind whenever there is a nightmare, Madhusudan during a tragedy, Narsimha while in a forest, the form of Shri Vishṇu in the celestial ocean when threatened by fire, His Varaha (One-tusked wild boar) Incarnation when there is fear of drowning, Raghunandan when there is fear of falling from a mountain and Madhav while performing daily chores.

From the examples given ahead, we will realise why a specific Name of Shri Vishṇu is used for a specific task.

A. Prajapati created the universe; hence, during a marriage ceremony, chant for some time the Name of Prajapati, who is responsible for procreation.

B. During war, chant the Name ‘Chakradhar’ (of Shri Vishṇu), who holds a Sudarshan-chakra (A spinning, disc-like weapon with 108 serrated edges) in His hand.

C. Lion is the kind of the forest and hence, chanting the Name of Narsimha (The man-lion Incarnation) protects us.

D. Shri Vishṇu appeared as Varaha to rescue the Earth from drowning in water and placed it on the head of the serpent Shesha (Divine serpent on which the earth rests). Hence, whenever there is fear of drowning, chant the Name ‘Varaha’.

Reference : Sanatan’s Holy text ‘Worship of Deities : Vol. 2 – Shri Vishnu’