As far as I know, BNB is used for the Binance exchange platform and is used for transaction fees, trading fees, Pay, etc. which are utilities of BNB. As far as I know, if you use BNB, the trading fee becomes smaller than average.
There are also some users who do not use BNB as a utility on the Binance exchange, maybe there are other needs that make the factor not to use BNB coins, for example, the ease of arbitrage between centralized exchanges or with DEX.