It could be a simple wrist strap or a full fledged sling similar to a camera strap.
Our highly adjustable phone sling keeps your phone close to hand, safe and out of the way. This sling makes your device even more versatile so you’re enabled to do what you need - hands-free.
www.mous.co
A friend who takes a lot of rare plant photos with his phone for his botanical guide books always uses a lanyard so he can go hands free quickly while he’s working, taking notes, collecting samples, etc.
I have another friend who uses his phone for taking photos while fly fishing. You can imagine that it’s a good idea to have a strap on your phone when fumbling with a fish and a fly rod while trying to take a photo over the water.
Those are just a couple of examples of how they can be useful.