To construct a reachability tree we start with the initial marking and then consider all enabled transitions and fire them one by one. That is, we find the marking corresponding to firing each.
I can't see why is this meaningful because if we were simulating the petri net then starting with the initial state we would fire all possible transitions that can fire together to reach the next marking and not just one of them.
So I don't understand what's the purpose of constructing a reachability tree if the markings (configurations) it will give consider firing only one transitition at a time which isn't how a petri net works.