Camera bags offer protection and convenience.
The camera bag is more than just a "purse" for a photographer. It can mean the difference between a great shot and a missed opportunity. There are both obvious and obscure items that a photographer can have on hand to make life in the field easier to manage, things that evolve into a photographer's "style" when capturing his vision.
The Essentials
Camera bags are a great home for extra lenses, flashes and other basic items used with your camera system. Extra batteries are critical with contemporary electronic cameras. Without power your camera is useless, so having a charged backup battery at hand at all times is simply necessary for successful shooting. Use compartments or cases to keep equipment from getting jostled. Thick socks make excellent lens protectors in a pinch. Extra memory cards (or film, if you're old school) cover card failures as well as extra image storage.
Photographic Accessories
These items extend the range of your camera system. Consider a mini tripod. While not a replacement for its big brother, it has many uses for increasing camera stability. As well as packing filters you commonly use, pack filter wrenches so your shoot isn't compromised by tight filters. Rubber pads made to grip jar lids are excellent for this purpose as well, and take up very little room. Lens cleaning supplies should be on hand at all times to keep optics sharp for best performance.
General Tools
The camera bag could be described as a photographer's do-everything Swiss Army knife, so the inclusion of a literal Swiss Army knife or other multitool is a natural. Add a small pair of pliers if your multitool doesn't include them. A small screwdriver set often comes in handy, as do small rolls of tape of different types and size. Zip-locking freezer bags can be improvised into many uses, from storage to camera rain gear. Avoid sandwich bags, as the plastic is thin and easily torn.
Human Considerations
An inexpensive pouch poncho is a wise addition to keep shooting in unexpectedly inclement weather. Likewise, articles like gloves and sunglasses can make dealing with the elements easier. Water and energy bars may be prudent additions if you are on a photo hike, and a length of strong cord often comes in handy. A small first-aid kit may never get used, but it will seem like a master stroke if you are in need.
Other Ideas
What you're shooting and where may give you other ideas for items that will make your outing successful. If you are trekking into a remote area, using your camera bag to carry basic survival items is smart. Close to home, you can pack light. In fact most photographers have a variety of camera bags to meet different needs. Pens, pencils, markers and a small notepad are useful additions.
Tags: your camera, Army knife, camera system, comes handy, comes handy small, hand times, handy small