What they're gaining is interoperability with public blockchains. JP Morgan's permissioned blockchain—Quorum—is essentially a fork of Ethereum. They can issue assets on their private chain (which, yes, by itself is no better than a traditional database). However, they can then transfer these assets to and from public blockchains.
By doing this, they are gaining access to the greater crypto ecosystem while still maintaining centralized control when they need it.
Here's a little example of how this might work in practice:
1. You hold $100k in your JPM money market account
2. You convert $25k of that to JPM's stablecoin on their Quorum chain
3. You withdraw this $25k JPM-coin to the public Ethereum blockchain
By doing this, they are gaining access to the greater crypto ecosystem while still maintaining centralized control when they need it.
Here's a little example of how this might work in practice:
1. You hold $100k in your JPM money market account
2. You convert $25k of that to JPM's stablecoin on their Quorum chain
3. You withdraw this $25k JPM-coin to the public Ethereum blockchain
4. You use it in Ethereum's burgeoning "Decentralized Finance" or "Open Finance" protocols.. some examples: https://www.dharmalever.com, https://compound.finance, https://www.augur.net/
5. You finish what you're doing, and transfer it back to your JPM account