When the driver is not driving, its output will be high impedance. Because of this and the bus differential termination resistance the voltage between pins A and B will become 0V. In the RS-485/RS-422, 0V is an undefined state. In an undefined state, the receiver might output the wrong logic state or might oscillate. Pull-up and pull-down resistor are required if your application always requires that receiver be in a defined state. It is not mandatory that you have them, but it is generally a good idea to include them. This is known as external fail-safe biasing.
Generally, the value of these resisters will be between 100-1kΩ depending on the applications. If you are using short cables in your application, you will only need to place external fail safe biasing at your master; however, if you are using long cables, it is best to place external fail safe biasing at both ends.