Convenience, plain and simple. I've never once been concerned about battery life on my phone. It's VERY rare for me to ever be below 50% at any point in the day, even on a long day where I've been mostly on cellular. But the iPhone now has a feature that even if your battery is "dead", some cards will still be available for use (it's a very specific feature/setting).

Same applies with my watch, which has the same cards available. It's nice if I run to a convenience mart after a run, where I often don't carry my phone or wallet. Just tap my watch at the terminal, and off I go!

I carry -- and regularly use -- around 5-6 cards that all have specific uses. Having a digital wallet to manage these is awesome. My physical wallet, I carry only three cards: my ID, debit card and my health insurance card.