Why should a payments business have more than one banking or liquidity partner?

Because growing transaction volume does not help you when the only route you have stops working.

I put a lot of emphasis on redundancy. If you depend on one bank and one liquidity provider, losing either relationship can interrupt the entire business. You may still have customers, demand and a profitable product, but no practical way to complete their payments.

Opening another account after the problem starts is a poor recovery plan. The replacement provider still has to understand the business, complete its checks, agree the commercials and get the integration working. Meanwhile, customers are waiting.

My preference is to build additional working relationships while the business is healthy. Start with your first provider, then establish a second and, where the economics justify it, a third. That applies to banking, conversion and payout arrangements, depending on where your dependencies sit.

I also prefer backups that actually process transactions. Where your agreements and approvals permit it, route some business through the additional providers so the operational relationship stays active. In a local-collection model, that might mean distributing transactions between approved providers rather than leaving one completely unused.

There is a cost to maintaining redundancy, and you should include it in the business model. But compare that cost with being unable to transact while you rebuild an essential relationship. The objective is to make a provider failure something you can manage while continuing to serve customers.