Can someone explain why someone would hold a stable coin?

I'm struggling to understand why anyone would hold a stable coin. If a stable coin is just a proxy for holding a dollar why not hold a dollar instead. If you hold a stable coin you're holding an asset backed by US treasuries so you're holding a dollar proxy which won't gain nominal value which is backed by something that has risk (treasuries). If you want to hold something that's valued at a dollar why not just hold a dollar and and if you want to hold the thing stable coins are backed by why not just hold treasuries. Am I missing something?