When I first started calling coyotes I didn't "need" a butt pack, backpack, or anything to carry stuff in. I carried three mouth calls, my rifle, and extra ammo was carried in my coverall or pants pockets.
Then I bought a FOXPRO ecaller. I purchased a small butt pack and carried the FOXPRO in that and the remote in one of my coverall pockets.
Next, I purchased a digital camera that I wanted to carry with on my hunts and needed a bigger butt pack for that and my FOXPRO.
A while later I decided to purchase a Leica 1200 range finder and needed to carry that along too. I also felt I needed some room to carry extra batteries for the caller and remote, a bunch of the custom-made calls I had purchased also needed to come along on my hunts, and for hunts when the weather was still somewhat warm in the fall, I needed room to carry some liquid refreshments. That necessitated purchasing a backpack.
I have a desert camo backpack done in the digital pattern for fall hunting. My wife made a white cover to go over that backpack for when I am out in the winter in the snow. I haven't weighed that backpack, but it is plenty heavy, but it is handy to have all that "stuff" along when you need it.
I also carry along a roll of TP, face netting, my Caldwell Wind Wizard, extra knife, tweezers to pull out the cactus stickers, some cookies or even a sandwich if I am going to go out for a long hike. I carry the round base for my tripod that I use to set my FOXPRO up on to get it off the ground and improve the communication between it and the remote. In the summer when I am out prairie dog shooting, I carry a couple containers of water as well as lots of extra ammo.
The backpack is probably not the handiest to get in and out of in a hurry, but I have been able to get my rifle up and pick off a few (not all) of the coyotes that I have scared up while walking in to a calling spot. I am going to start carrying my S&W Model 686 .357 Magnum revolver along when coyote hunting because of the cougars we have up here in North Dakota. I'll carry that revolver in a holster on my belt so a butt pack would make it extremely difficult (if not impossible) to get that revolver out in a hurry, so I'll most likely stay with the backpack.