There's a reason to temporarily move from say BTC to USDT: If you think bitcoin/crypto currency is going to trend downwards for a bit and your exchanges don't have fiat balances (so the vast majority of them?) you sell your cryptos for USDT, wait for cryptocurrency prices to fall, then re-trade your USDT for more BITCOIN or whatever. Then you have accumulated more bitcoin (the premise is that you think it'll go up again).
Edit: ninja'd.