It is mandatory that your broker fills you at the best possible price for your orders.
Under normal market conditions (e.g. normal liquidity, no important news releases or surprise events), there really is no reason for your broker to not fill you at, or very close to, the market price you see when you click the “buy” or “sell” button.
For example, assuming you have a stable internet connection, if you click “buy” EUR/USD for 1.3000, you should get filled at that price or within micro-pips of it. The speed at which your orders get filled is very important, especially if you’re a scalper.
A few pips difference in price can make that much harder on you to win that trade.