For espresso you want to be able to dispense the same amount of coffee directly into your portafilter every time, a timer grinder allows you to do this. You can usually set the grinder up for single and double shots by adjusting the timer, personally to keep things simple I just stick to the double basket in my portafilter and always use the button for a double shot on the grinder. My wife likes this too as she doesn't want to have to start weighing out coffee, that just frustrates her!
The disadvantage is that a timer grinder costs more than a manual grinder usually.
