Why is it recommended to sprinkle water on the graves?

Hazrat Aba Abdillah (Imam Jafar As-Sadiq) (peace be upon him) was (once) asked about the secret behind sprinkling water on the graves?

His holiness the Imam AS said, ‘till the time the soil of a grave is fresh and wet, the punishment (Adhab of Allah SWT) is removed from the grave (as a mercy from Allah SWT).’

In another Hadith narrated from Imam Sadiq (a) and his noble father (Imam Muhammad Baqir) (a), he said that the grave of the Prophet (PBUH&HF) was raised by a span of a hand (from the ground) and he (The Prophet PBUH&HF) has advised (the Muslims) to sprinkle water over the graves.

Ref: Hawzah.net