Firstly I don't claim to be a devotee of these lords. But in the last 5000 years so many devotees of Shiva and Vishnu appeared in India and they write in their respectful works, the benefits of devotion and the possibility of obtaining moksha through devotion (bhakti) and surrender (saranagati). Check out the lives of Nammalwar, Thirumoolar, Ramalinga Vallalar, Andal, Agastya, Samartha Ramdas, Tulsidas (to name a few out of thousands) whose works and recordings have left a lasting impact on the cultural fabric and ethos of Bharat (India). According to Sadhguru Jaggi Vasudev, devotion means to be devoid of one's egoistic self and being completely surrendered to the will of the Divine and doing only what is needed to grow in spiritual life and accepting with humility and gratitude what comes naturally to him. Indeed without "devotion" one doesn't achieve anything useful in life, even on the material platform. Let's say if you want to excel in mundane activities like playing any sport, you have to be completely "devoted" to that sport to become a successful world class player.
Forms of Shiva and Vishnu are personified and localized representations of Parabrahman (a.k.a Supreme Absolute Truth / Cosmic Consciousness / Vast Inifinite Grace Light of Compassion - ArutperumJothi ) that creates, maintains and destroys millions of universes. Everyone in the material universe is affected by varying degrees of the six enemies of the mind (Shadaripus) which are Lust or desire – Kama, Anger – Krodha, Greed – Lobha, Delusion – Moha, Arrogance or pride – Mada, Jealousy or envy – Matsarya, so it is most intelligent to surrender to that ArutperumJothi and appeal for liberation as no one else in the cosmos is capable of providing the boon of liberation as they are all stuck in the prison of this material universe. The Supreme Absolute Truth (ParaBrahman or ArutperumJothi) whether worshipped through forms like Shiva, Vishnu (Sarguna Brahman) or formless (nirguna Brahman) are not like some Zeus or other Greek mythological personality that requires devotion of mortals for their sustenance. The parabrahman (Arutperum Jothi) according to vallalar is indeed the very source of all sustenance and it is very eager to bestow moksha and all other boons, if humanity is ready to give up their material ego concsiousness and walk the divine path and live a virtous life of (JeevaKarunya) compassion, kindness, mercy and love. After attaining moksha one becomes one with the divine (parabrahman) just as a drop of water merges into the ocean after which there is no difference or inequality. The liberated souls get an eternal grace light body (SatChitAnanda Swarupa) that is similar to the Divine and enjoy all mystic opulences and freedom to travel to any universe and any dimension at will. Just sharing my humble few cents of information that I gathered