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

Vishnusahasranam is a famous stotra (A hymn of praise) of ShriVishnu. Amidst recitation of these thousand Names, ShriVishnu is ritualistically worshipped with an offering of tulsi (Holybasil) leaves or lotus flowers. Although, all the Names mentioned in Vishnusahasranam are ShriVishnu’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.

औषधेचिन्तयेद्विष्णुंभोजनेचजनार्दनम्।

शयनेपद्मनाभंचविवाहेचप्रजापतिम् ||१||

युद्धेचक्रधरंदेवंप्रवासेचत्रिविक्रमम्।

नारायणंतनुत्यागेश्रीधरंप्रियसंगमे ||२||

दु:स्वप्नेस्मरगाेविन्दंसंकटेमधुसूदनम्।

काननेनारसिंहंचपावकेजलशायिनम् ||३||

जलमध्येवराहंचपर्वतेरघुनन्दनम्।

गमनेवामानंचैवसर्वकार्येषुमाधवम् ||४||

Meaning : Contemplate on ShriVishnu while taking medicines, Janardan during meals, Padmanabha while sleeping, Prajapati during a marriage ceremony, Chakradhar (ShriVishṇu / Shrikrushna) during war, Trivikram during a journey, Narayan 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 ShriVishnu in the celestialocean 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 Vishnu 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 ShriVishnu),who holds a Sudarshan-chakra (A spinning, disc-like weapon with 108 serrated edges) in His hand.

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

D. Shri Vishnu 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’