Around five hundred years ago, the land of Navadvipa (Bengal) was considered to be the capital of scholars. As a result, these scholars were either atheists or were engrossed in following the rituals of karma-kanda as per the Vedic scriptures. Pure devotional service to the Supreme Personality of Godhead Lord Sri Krsna was absent. Seeing this, Advaita Acarya's heart over flooded with compassion. He appealed to the Lord in different ways calling Him to appear so that He could establish dharma Himself. Suddenly he remembered a verse
tulasi-dala-matrena jalasya culukena va vikrinite svam atmanam bhaktebhyo bhaktavatsalah
"Sri Krsna, who is very affectionate towards His devotees, sells Himself to a devotee who offers merely a tulasi leaf and a palmful of water." The Lord thinks "I don't have anything with Me which can be equated with tulasi leaves and water. I must give Myself in charity to repay this debt."
Srila Prabhupada writes in this connection – The Supreme Lord is causelessly merciful upon His devotee, so much so that even the poorest of men can offer Him a little water and a flower in devotion and thus please Him. (SB 5.3.6 purport.) In this way, meditating on the lotus feet of the Lord, Sri Advaita Acarya started offering holy Ganges water and tulasi manjaris to the Lord. And hearing his intense cries the Supreme Lord appeared in the form of Sri Krsna Caitanya Mahaprabhu and established the yugadharma – Harinama Sankirtana.
Among all articles which are offered to the Lord, Tulasi is most highly praised. Therefore one may perform perfect worship of the Lord by simply offering pure water and Tulasi leaves. According to the Garuda Purana, Tulasi must always be offered on the naivedyam.
Puja, bathing, and offering of food and drink performed withour Tulasi cannot be considered as puja, bathing, and offering to the Lord. The Lord does not accept anything or drink anything that is without Tulasi. -Brhan-naradlya Purana During the puja, one should offer T ulasi leaves to the Lord's feet. Tulasi garlands may also be offered around His neck.