In Padma Purana , Uttarkhand chapter and in Agni purana it described about the secret behind churning mandarachal mountain. Lord Vishnu actually plotted this plan not to save Devas but for some other purpose.
Indra gets devoid of everything
Once sage Durvasa presented a garland of Parijata flowers to Indra. Indra was on elephant – Eravat as he was traveling somewhere. So Indra kept that garland on the forehead of his elephant-Eravat. Soon the garland slipped and fell down and got stamped by the elephant. Watching that Sage Durvasa got angry.
Soon sage Durvasa cursed Indra to become devoid of every thing Indra had. His words came to be true. Demons conquered Indra and took his kingdom. As The deities too involved in the management of the cycle of this Kala chakra, since deities became poor, all the three lokhas also became poverty ridden and filled with grief. Indra then requested Brahma to help him. Brahma suggested to take Vishnu’s help.
Lord Vishnu’s secret behind the Plan
Life Cycles of different worlds are dependent on each deity as each one has their own part in running specific karma’s of every living being. Because of that whenever these deities are being troubled all the three worlds which are controlled by these deities are always in trouble.
So Lord Vishnu then made a plan to solve this issue permanently. The plan is to make deities very powerful and immortal so that no one can destroy them and in turn all the three worlds would remain safe.
Lord Vishnu’s Plan of Churning Mandarachal mountain
Lord Vishnu then told about churning Mandarachal Mountain with the help of snake Vasuki. He told things will emerge out of it which will benefit them (devas) as well as all the three worlds and it is a permanent solution.