Why cold storage? Because security:
For security purposes, Square stores a reserve of Bitcoins in an offline setting. By having these funds offline, we reduce attack surface and hence risk of theft.
Square can move the funds offline at any time, but moving them back online requires a multi-party signing ceremony. They can also embed programming logic into the cold storage modules, so that only Square-owned addresses can receive the funds. That’s defense-in-depth, right there.
Bitcoin’s latest bull run is over, but those who believe in decentralized money continue to toil away… building the future they want to exist.