It is handy to use when a darkroom is not available as is often the case in field shooting. It is also used in commercial photoprocessing labs, often to change paper.
A changing bag has two sleeves at one end for both the user's arms, and a zipper (often more than one, for double layered changing bags) to insert the tools and film needed. There are several sizes available, from smaller ones for many still photography applications to larger bags used in large-format still photography or filmmaking, which may need to hold both a magazine and a can of film stock which each have a 1000 foot capacity. Larger changing bag sizes are also available as "changing tents", where the top of the bag can be held in a dome-like configuration through the use of two curved rods.