What is the explanation of this Hadith, and what is its degree of authenticity?

The agreed-upon Hadith narrated by Bukhari and Muslim mentions that when the Prophet, peace and blessings be upon him, went out to relieve himself, Anas and another young boy would follow him with a vessel of water for purification (istinja’) and an ‘anaza (a short spear). There was disagreement regarding the reason for carrying the ‘anaza, but Al-Hafiz favored the view that it was used as a sutra (barrier) for the Prophet, peace and blessings be upon him, for prayer after relieving himself and performing ablution. Imam Bukhari titled this Hadith "Chapter on Purification with Water" to refute those who disliked purification with water or denied that the Prophet, peace and blessings be upon him, performed it. Among the benefits of this Hadith are the permissibility of using a young person (for assistance), purification with water, and it indicates the superiority of purification with water over using stones.