I use it when I want to finish rendering something that I cut short . When I open a half rendered image it automatically continues from where I left off. For me this is great, often I cant be arse'd to wait for a complete render so I do it in stages, it also gives me a space in which to decide if the composition etc is working out. It pays to put an image to one side for a while and then re-appraise it when I am less giddy with my supreme creative genius! Create when drinking! Final render when sober! Now you know why I always say CHEERS!