I still don’t follow why the second currency is required? They will always swap a UST for $1 of LUNA so a UST is implicitly worth $1. LUNA itself doesn’t have

I still don’t follow why the second currency is required? They will always swap a UST for $1 of LUNA so a UST is implicitly worth $1. LUNA itself doesn’t have anything behind it though apart from concidence. So we have a stablecoin manufactured from a volatile unbacked asset where the market cap can fall arbitrarily low. I know it failed, but it’s not quite clicking for me what they were even trying?