They aren’t mandated to choose a formal leader (and they didn’t have one prior to Bob Brown stepping up in 2005). My guess is that the choice to have a leader is a reflection of their growth as a party. For better or worse, coverage of Australian politics is often based around how the individual leaders of each party are responding to current events. Having an official leader elected by the party room allows The Greens to put someone forward in direct competition with the major parties. Bandt losing his seat made his position as leader untenable, because no one is going to take an unelected leader (particularly one who has lost their seat) seriously. Furthermore, losing his job also likely means he loses the financial ability to be a full time leader, not to mention he is no longer privy to much of the information that would be necessary for him to effectively lead his colleagues in Canberra.