Live accounts have instant execution which includes the use of requotes, while STP/ECN accounts have market execution without requotes. In addition, there is a difference in spreads due to the different liquidity provider.
There are 3 advantages to using live accs:
1- Brokers can give fixed spreads, making costs more predictable (and also smaller) for the trader.
2- Commissions/fees for trading also more advantageous.
3- The broker has greater flexibility in offering additional features like bonuses, promotions, and leverage options.
You can see all of this in effect: STP/ECN have only 1:200 leverage, while live have up to 1:1000. Commissions/spreads also lower, and there are also no bonuses on STP/ECN account types so...