A sling bag is handy for things you want to move around with you as opposed to carry, if that makes sense. I wouldn't want to hike ten miles with a heavy sling bag but it's a good format for something to wear for a hundred yards walking to a vehicle and tossing it in the back seat. If you're toting to the office or to a vehicle they work well. If you're hiking or walking all day a sling bag isn't as good as a backpack but works if you keep the weight to a minimum.
“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman