In embedded field they are basically used to set a default input/output values. Suppose you have set a pin as input and you are trying to read the value of that pin before applying any input device on that pin. Such a case is called high impedance state and you can’t predict wheather the value read will be logic high or logic low.
Here if you were using pull down resistor to the pin, then eventhough no input is connected, the pin is read as logical low .If pull up were used then the input pin will read values as logic high eventhough no input device is attached.
It is same with the case of output pins and output devices