Bitwise operations are slightly faster than addition and subtraction operations and usually significantly faster than multiplication and division operations. Because bit wise operation is performed with in one register, and other arithmetic instructions needed to handle more then one registers.
So a number shifted left by one bit means, number multiplied by 2.
num << 2 is equivalent to num * 4, but first operation is faster than second one
Suppose interviewer asked how to fast multiply a number by 9?
(num << 3) + num
This is same as
(num * 8) + num = num * (8+1) = num * 9