Why do you think that there shouldn't be a destination tag for BNB on the BEP20 network? I am not that familiar with centralized exchanges and different altcoin features, but according to my undertanding, the memo makes sense if an exchange uses the same receiving address, regardless of the network. Does it not?
Memo is not used with Binance Smart Chain as I know. A most famous cryptocurrency that uses memo is Ripple and there are other altcoins with memo for transactions too. Exchanges usually have their warning messages that if you don't add Memo for your withdrawal information, it can be lost and their exchanges won't be responsible for your loss.
I said Ripple is a most famous because it's likely an oldest and top altcoin with Memo.
