In terms of what does a deployment bag do? It keeps your recovery system neat, tidy and organized, aiding in a neat, orderly deployment.
If you look at a lot of altimeter dumps, you can see the recovery system deployment, in the form of a spike that's actually a higher G load than when the motor first hits. This happens when the canopy is allowed to inflate before the lines or riser are taught.
A proper deployment bag helps prevent this by causing the lines to unstow first, then the canopy is allowed to come out and inflate. This means the recovery system is stretched out, and the canopy inflates more slowly (still a matter of seconds), causing it to not snatch the rocket hard. If you see a really hard deployment, you can watch a 20+ pound rocket bounce back on the end of a harness -- NOT a good thing!
The other thing a deployment bag does for you is reduce the risk of an entanglement or an inversion.
They're all about reliability, and it's never too early to start learning about them or using them. I have a friend who uses them on 5lb rockets.
-Kevin