Servos are just one kind of motor. There are many other types... Servos have a built in H-bridge so you don't need an external one.... The h-bridge you are referring to is used to drive basic dc motors (not servos)... Remember a dc motor has only 2 wires and no electronic control board in them. So you will need an h-bridge to control them properly in order to move your robot forward, reverse and for speed control as well... You can't do much with a dc motor if you just hook them up directly to a battery... The motor will just spin at a constant speed and in one direction. You won't be able to control your robot... Use an h-bridge and now you can not only control the motors speed but it's direction as well...

For example Roli uses an h-bridge because the motors that drive his treads are 2 geared DC motors (not servos)....

Richard. Thanks for the great explanation.