> Do you hold deposits for others that they can withdraw at any moment?

Yes you do actually. There are many withdrawal events such as taxes, medical expenses, vehicle repair, and others that you need to provide liquidity for in short notice. If your funds are locked into treasuries or other assets that have a low current value relative to their final worth like SVB then you will have to sell them at a loss which might ruin you financially. Unfortunately there's no FDIC for individuals.