Lightning and liquid are completely different, they are not the same. However liquid and lightning used together intelligently, allow continual, non-custodial exchange and savings of Sats even when fees are high.

In wealthy countries maybe it doesn't matter... people will pay the on-chain fees regardless once they figure out what Bitcoin is.

But in relatively poor or developing countries, the fees will pile out of control in my opinion. Liquid is then used to manage liquidity... or to accept lightning payments through BTCPay server just long enough to have enough Liquid-Sats (a few hundred dollars worth) to open a Lightning node. Lightning QR -> Liquid (BTCPay Boltz works like this as I have detailed above).

That's the purpose.