The concept goes back to musketeer times. Devices called Apostles
http://www.northamptons.org.uk/musket.html
were basically bottles or containers with measured charges in them.
Later, rolled (wax) paper cartridges
http://www.researchpress.co.uk/firearms/britain/enfield/enfieldcartridge07.htm
were carried in cartridge boxes
http://www.gggodwin.com/page2.htm
became the dominant method of carrying premeasured charges.
A modern carrying device is the EZ Loader listed above which works well on halfstocks, but do not work at all on some fullstocks. Another is a piece of clear plastic tubing plugged at both ends. Pull the plug from the powder end and dump it into the bore. Then pull the other plug and push the ball out with your ramrod and then into the bore. There are commercial (color coded by caliber) Kwik Loaders that work similar in concept.
Yes, there are several advantages in using premeasured charges. You do not need a powder measure, you have several containers in case one should become damp,(All of your powder is not in one basket) you can load more quickly, and you do not have to remember what charge goes with this situation.
Finally, I have gone into the woods and heard an odd sound -- sort of like dry rain. I would look down and find that my powder had escaped its confinement. Now, I carry both.
Now the question is, what period do you enjoy? This should drive your choice in how to carry your loads.
Are you sick of all of this yet?
I am making a small belt mounted cartridge box that will carry 5 hunting loads and can double as a shotgun shell carrier. It will have a replaceable block for my many sizes of calibers. I range from .40 to 11 gauge.
(Uh ohh -- I mentioned the unthinkable!!! -- I am swiftly pumping out my hundred pushups now !!!)
<G>
YMHS,
CrackStock