Supplications (duʿā) has been called the essence of worship for two main reasons. First, by supplicating to Allah, a person fulfils his obligation of calling on Allah, which is understood from the verse “And your Lord says, ‘Call on Me, I will answer you (your prayer)’” (Qur’ān 40:60). This is worship in its purest form. Second, by invoking Allah, one realizes that only He can fulfil one’s needs. The servant who calls upon his Lord abandons hope in everything else and turns his full attention to Allah, bumbling himself in front of Him. This is the essence of worship. Another desires result of worship is to attain reward from Allah. Because there is such great reward in supplication, it has been called the essence of worship.
(Mazād al-Rāghibīn, pg. 7)