When Rama learnt that Sita was captive in Lanka he needed someone to cross the ocean and confirm the news. Jambavan the wise bear asked Hanuman to do the needful. When Hanuman said that he did not have the capability to leap over the ocean, Jambavan narrated to him the tale of his birth. Punjikasthala was an apsara who was condemned to live one life as a monkey. She was born to the monkey king Kunjara as Anjani and married the monkey Kesari. Anjani had the capacity to revert to her apsara form at will. Once she was roaming on a mountain top in her apsara form, when Vayu the wind demi-God brushed against her and lifted her garment. She protested that Vayu was violating the virtue of a married female. Vayu said that he meant no offence. By brushing against her he had impregnated her in a non-physical manner. He told Anjani that she will give birth to a valiant son who will have powers equal to him. After narrating the tale, Jambavan said that Hanuman possessed the powers and asked hanuman to make the leap.
Despite the erotic touch to the encounter between Vayu and Anjani, the fact of the matter is that destiny was playing out itself. Hanuman was destined to leap over the ocean and therefore he needed the power to do so. Only Vayu could have given him that power and hence the destined encounter between Vayu and Anjani.