Here's an excerpt of an article on Forbes listing the advantages of using a holster case.
Your giant cellphone is less likely to break from you sitting on it if it’s in your holster. Although phones are reinforced now so this won’t happen, it never would have happened for me, because the pressure from my jeans or weight won’t cause the millimeter-thin steel casing on a phone to bend.
You are less likely to take out your phone when you sit down and leave it somewhere if you don’t have a giant uncomfortable phone in your pocket. Wearing a holster means I’m not inclined to take out my phone when I’m doing gymnastics or rock climbing or shark wresting or just sitting in a low couch.
Fashion is always changing. Sometimes I wear shorts where a holster would be the only obvious place to keep the phone. Now, this isn’t every day, but who knows? It’s getting hot outside. Sometimes you’re running with friends at the beach in the waves and a holster is the only way to keep your phone from getting wet.
But mostly, it’s because a holster is just the most common sense solution to carrying your phone. It’s more comfortable than the pocket; it’s more likely to get your attention with an urgent notification than if it were in your purse or murse, and it’s much easier to get access to in meetings.