The Sunder Kand has innumerable benefits, some of which are mentioned below.
Growth of devotion for Lord Hanuman. Sunder Kand, a selection of Hanuman’s divine plays on Earth, can be recited as a distinguished prayer to Lord Hanuman. Its recitation may develop bhakti (devotion) towards Lord Hanuman, who can grant the bhakti of Lord Rama (God), leading to eventual liberation. As I have already summarized on this page, Hanuman “is worshipped among Hindus for his delight in connecting jivas [individual souls] to God.”
Growth of devotion for Lord Sita-Rama. Reading about Hanuman can develop bhakti for Sita-Rama. As mentioned in this post on my blog, “we can learn from [Hanuman’s devotion for God] and add bits of selfless service in our own path of devotion.”
Protection from sufferings. Hanuman can protect beings from all kinds of sufferings, including physical, mental, and spiritual ones. One of the main messages from a popular prayer titled “Sankatmochan Hanuman Aashtak” is that when Hanuman has the potential of even rescuing Lakshmana, Devi Sita, and Lord Rama, (there appears to be a figure of speech here, given that Lord Rama is the Supreme God himself), what can stop Hanuman from removing the suffering of a simple human being? Along these lines, the Sunder Kand supposedly has a protective effect on the beings who recite or study it. Note that reading this prayer may antidote many bad karma of the past as well.
Astrological Benefits. Like the Hanuman Chalisa, the Sunder Kand has astrological significance. An astrologer may recommend the Sunder Kand to nullify one or more malefic planetary infleunces (if applicable) — from Mars, Saturn, Rahu, and Ketu.
Recognition of Lord Rama’s grace. Lord Rama has given Hanuman a major role in his divine play, the Ramayana, demonstrating Lord Rama’s love for all his devotees. Sunder Kand celebrates this grace of Rama on beings in his creation.
Source: My answer to a Quora question on this page.