Why is it necessary to check for extraneous solutions in radical equations?

Extraneous solutions mean "extra and not necessary"
Example: the area of a square = 16, what is the side of the square:
:If a is the side, the area is a²:

a² = 16, the square root of a = +√16 or a= - √16 :
 a =4 or a = - 4 , Obviously you can't have a side of  - 4, then this solution is an extraneous one