Solat Tahajjud : Make an intention before sleep. WHY?

Upon going to sleep, one should make the intention to perform Tahajjud. Abu ad-Darda'(radiallāhu‘anhu) quoted the Prophet (Sallallāhu 'alayhi wa sallam) as saying: "Whoever goes to his bed with the intention of getting up and praying during the night, but, being overcome by sleep, fails to do that, he will have recorded for him what he has intended, and his sleep will be reckoned as a charity (an act of mercy) for him from his Lord." [Narrated by an-Nasā'ie and Ibn Majah: Sahih]