The Fundamental Difference:

Binance Spot Wallet (Spot Trading): Your coins are on the Binance platform, and the platform is responsible for holding them. You do not own the private keys.

Binance Web3 Wallet: Your coins are directly on the blockchain, and you own the private keys.

"Can I sell at a high and get USDT in return?"

Yes, absolutely. If the BOB you purchased is in your Spot Wallet on Binance.