It can be for any reason: being it because they are sounding great live, or they seem to have fun, or because It was innovative, It was emotional or was somehow very important for their career.
My favorite Stray Kids' encore is the Thunderous one they invited BTOB for after both competing for #1. They became closer that year due to Kingdom and they've been friends since then. From time to time we have updates of members going out or/and traveling together.
So It was very cute and fun