When you hold Tether, you're taking a risk that your Tethers might not be redeemable in USD. As far as I know, this risk isn't compensated in any way, shape or form.
When you deposit your Tethers somewhere, you're taking an additional risk, namely the risk that you might not get the deposit back. The interest that you get on deposits is compensation for taking that risk, but not for the other risk.
For sure, holding tether (or any stablecoin) without investing it is pointless. It can't go up, but might go down. The only reason to hold it (longer than to do a transaction) is if you're making a return.
When you deposit your Tethers somewhere, you're taking an additional risk, namely the risk that you might not get the deposit back. The interest that you get on deposits is compensation for taking that risk, but not for the other risk.