Why bridge rectifiers are used in case of DC power supply.?
As Vladimir Cravero mentioned in the comments, using a bridge rectifer in a DC application would be make the input polarity insensitive. 

The image below is a single diode used for reverse polarity protection. If V1  &lt; 0.7, R1 sees no voltage/current, and the load is protected.







simulate this circuit &ndash; Schematic created using CircuitLab







simulate this circuit

In the image below, if the V1 is wired correctly and V1 > 1.4 , then D1 and D2 conduct. 

If V1 is wired incorrectly, meaning that + and - are accidentally reversed and V1 &lt; -1.4, then D5 and D3 conduct.

When V1 is between +/- 1.4, there is not enough voltage to overcome the diode forward voltages, so R1 sees no voltage/current.

So while this does protect against polarity reversal, I like to think of it as polarity insensitive at the cost of a reduced output voltage of 2*Vd.