Does leaving land alone actually help nature conservation?
I'll give you an example, because this question is pretty case specific. In Delhi, my hometown, they kept areas of the forest, the 'Sanjay Van' free from construction and people and only very recently started cleaning it up. As a result, an invasive variety of the Acacia tree (known locally as keekar) more or less took over the forest and has basically wreaked havoc on the wildlife. 
I think better than just letting Nature take it's course, humans should help get abandoned areas back to a sustainable point, where it can integrate with the wildlife, but again, it depends on each area.