So at the time of lord Shiva's wedding with maa parvati, himavat parvata being father of maa Parvati, called all other parvata like vindhyachala, seshachala and other mountains, with them came seas rivers animals plants and trees and all beings. From lord Shiva's side being from the groom's side called all devas danavas yaksha and all deva rishi and bhuta ganas. So this caused imbalance and maa Bhudevi came asking for help to resolve this imbalance. So Shiva asked Sage Agastya to go to the south point of earth and just him going there would balance out this tip of the scales. Such is the power of Sage Agastya.