There is great and profound wisdom behind this act of Prophetic tradition, praying in an open area of land. Muslims have annually two days where all people from every city meet each together, men, women, or children, and turn to Allah with their hearts in unity, praying behind one Imam, making takbeer, tahleel and supplicating to Allah sincerely together as if they were united upon the heart of one man while being happy and joyous about the blessing Allah has bestowed on them.

[From https://islamhouse.com/en/books/54235]