How to convert a standard bag into a camera bag
When you've spent a lot of money on a camera and accessories, you probably don't want to just chuck them in a normal bag. They are likely to get scratched, damaged, and it is also more difficult to find the accessory or lens you want when they are all jumbled up together.
A specially designed camera bag has padding to keep your camera safe, and also dividers to keep all your gear organized and easy to access. However, they tend to be quite expensive.
Many photographers also do not like using a bag that looks like a camera bag. It can draw attention to yourself that it not wanted when doing street photography. It can also draw unwanted attention from thieves.
To avoid these problems, you can take a standard bag, and add a padded divided insert. This allows you to use a normal bag as a camera bag. If you want to use the bag for something else, you can easily remove the padded insert.
Archival Clothing Field Bag by mrjonathanpotter on flickr (licensed CC-BY-SA)