Why Bring one? Where else would you carry your gear? In your shoe?

What kind should I get? Internal frame backpacks are very comfortable and stay very close to your body, thus, improving you balance. External frames, however, have more room and if yours is big enough, you can use it to block the sun. For a weekend trip, an external frame backpack should have about 1500+ cubic inches. An internal frame, should have 3500+ cubic inches.

figures retrieved from The Backpackers Field Manual by Rick Curtis