A sling sits across the chest rather than hanging off one shoulder, which is why it stays put while cycling, hiking, or moving through a crowd — no constant re-shrugging to stop a strap sliding down.
The crossbody position also keeps the bag in front of you and within reach, so you can grab a phone, keys, or a snack without taking it off. That's the practical advantage over a backpack for short trips and active use.
Sized as a compact daypack: enough for essentials without tempting you to overpack. Suited to travel, day hikes, cycling, camping, or everyday carry when a full backpack is more than you need.