"No inherent effect. Many Monk skills, especially spells related to healing, become more effective with higher Healing Prayers."
Healing Prayers is a Monk attribute most used by those wishing to heal themselves or their allies. The attribute improves the duration and the effectiveness of skills related to the Healing Prayers attribute. The Healing Prayers skills category contains a list of skills related to this attribute.
Healing Prayers is the core ability of most healers. Primary Monks can achieve higher ranks in Healing Prayers (up to 16) than Secondary Monks, due to the usage of runes — whereas a Secondary Monk could generally get at most 12 ranks (13 in PvE). An E/Mo with Energy Storage can make a viable healer with full ranks in Healing Prayers and Energy Storage, as can a N/Mo with similar ranks in Healing Prayers and Soul Reaping. A R/Mo would not make a viable dedicated healer, due to lesser energy regeneration. A W/Mo would at best be able to heal themselves, because of an even lower energy regeneration and maximum energy.
As for the actual Healing Prayers skills, they are mostly rather one-sided and uninteresting, but they serve their purpose. Their purpose, of course, is to heal and keep alive all your allies. Because of this, there are only 38 skills linked to this attribute. The vast majority of them provide direct healing, unlike Protection Prayers. Examples of these are Heal Other, Orison of Healing, and Healing Touch. All of these have their small differences, but in essence they all provide a large health boost to you and your allies. These skills are boosted by a high rank in Divine Favor, so many healers choose to raise both these attributes to high levels.
There are a few more interesting skills linked to Healing Prayers, however. Healing Hands and Healing Seed heal your target whenever they are struck by damage, while Vigorous Spirit heals your target whenever they perform an action. The skills Healing Breeze and Mending provide health regeneration instead of healing directly, and are the best ways a healing mage has to deal with health degeneration effects like Conjure Phantasm. Finally, the skill Infuse Health causes the healer to lose half their current health, healing an ally by this amount or more.
Healing Prayers lacks any skills to damage foes, or indeed affect them at all. However, this one-sidedness is welcomed in a party setting, where each party member fulfills a specific role. Indeed, late in the game, venturing outside towns is considered suicide without at least one healing monk to help you. The downside to this is that while any party needs a healing monk, a healing monk needs a party. They cannot venture out and do solo play, at least without a mass of henchmen to accompany them (though generally speaking, a monk usually gets into a party relatively quickly).