It wasn't used as cash. People were settling debts on the spot. Cash is a debt. Cash/currency is a claim to gold. It means if I have $35 I can exchange those dollars for gold. They have no other purpose. When you exchange gold for goods you are exchanging money for a consumable commodity. The purpose of currency is to allow money to be exchanged at a level that the market needs. Gold is heavy making transactions involving commodities more than an ounce or two in value, unfeasible. The same goes for Bitcoin but with settlement velocity. Because confirmation takes at least 10 minutes, the coupon has to move faster than that to facilitate trade. Lightning solves this with instant transactions that settle later during a channel close. The twist is that Bitcoin is faster and cheaper for larger transactions on-chain than smaller ones like gold.