I'm guessing that boom bags have been around for 30 years or so, prior to that we would remove the sail off the boom, take out all the batterns, flake, bag it and put it in the cabin. Then something happened, well numerous events even that led to the boom bag as we know it today. These include larger boats (larger sails) less crew to do the work (post RBT weekend sailing has never been the same), full length batterns, flashy bat cars that attach to the mast which need special tools to remove, lazy jacks, and plenty more I'm sure. The result is a main that now lives on the boom and hopefully under a canvas cover of some sort.
There are two main types, one with lazy jacks attached as per the main picture above, which the sail falls into and a zip on the top to enclose the sail when back at the dock. The other is a cover with the zip underneath that you put on the boom once the sail has been nicely flaked on the boom. I'm ignoring in boom/mast furlers this time, that is a full comment in itself.
As a sail cleaner, I can tell what type of bag you have and what condition it is in, and wait for it, an opinion on which bag/cover is best, I think you already know what I'm going to say.
Zips fail faster than you can get them repaired, zips are also not waterproof, so if your zip is on top of the boom, your main will be wet everytime it rains, it will go mouldy and you will have to call Vacuwash to clean it, remember it is also too big, which is why you left it on the boom in the first place. Also at the clew end there is the perfect hole for 'birdy numb numb' to squeeze in and produce more birdy numb numb's, one week is all it takes! A sail can be brand new, and in a week can look like, well s*it.
My suggestion it to attach the lazy jacks to the bottom of the boom, sailtie the main to the boom itself, move lazy jacks forward, then cover the sail with a one piece cover, zip or clips under, making sure there are no holes for BNN.
Another huge advantage it that you dont have that huge bit of canvas flapping while you are sailing.
The sails below were home to BNN for a short time, think what fell on the owners heads as the sails were raised. (both sails cleaned up beautifully)