If you took the equipment of the pack of any of your characters and had someone explain what sort of character they thought you were, what would they say? Beyond the basics of race and class, what sort characteristics and traits could be determined from a pack?
Presumably if your pack was full to bursting it could be assumed that your character was either big into looting and collecting, or perhaps your character felt that everything had great worth and could come in useful someday. Potentially if your pack had vials and ingredients, it could be assumed that you were into alchemy. Perhaps your bag was essentially empty and it could be assumed that you only needed what you could keep on you and didn't need anything more.
While it is a different way of looking at it, you could think of it this way. If your character was found unconscious and you were being looted, what would your belongings tell the people that found you? Are you a trophy collector with odds and ends of vanquished foes in your possession? Are you a scholar with books and vials of random substances? Are you an alchemist with a pack full of various ingredients and elixers? Are you ever-ready where your pack is full of items that may become useful at any moment, but not filled with clutter to inhibit your readiness?
While a backstory can tell of a character's history, a look into their pack may supplement that with a little more insight into their personality that may not be so evident at first glance. Perhaps the fighter who is courageous and hard-headed may keep a sentimental not from his/her family or an emergency vial of healing potion in-case he/she gets himself/herself in a sticky situation. It may be that an illiterate character might have a book to show an effort towards literacy.
Thoughtfully choosing what is in your character's pack could add an extra layer to your character that may not immediately be known to the party. While most characters are willing to share their stories of how they came to be together, there still could be quirks, traits, and perhaps even secrets that can be held within a character's pack.