Why convert degrees to radians?

I want to know whats the difference between them,what they do, what they could be used in different situations

They are two ways of expressing angles. One is not superior to the other. Some math formulas (and thus Unity functions) expect or output values in degrees, others expect or output values in radians.

Radians are directly related to pi, and can be thought of as naturally occurring units when dealing with trigonometry. Draw the radius of a circle. Curve this line segment to lie along the curvature of the circle. The angle between the start and end points is one radian.

Degrees are more arbitrary, but human beings think in terms of degrees, so both have a place in mathematics. I can’t remember why 360 was chosen, but methinks it had something to do with the style of architecture used by its creators, this branch of mathematics being a practical evolution. Simply, many numbers divide evenly into 360.

Which to use depends upon which is more convenient at the moment based upon what you’re doing. This knowledge comes with experience but it’s mostly intuitive; you either need one, or the other. You can convert back and forth between them by multiplying your value with a constant.

float degrees = 90;
float radians = degrees * Mathf.Deg2Rad;

float radians = 2*Mathf.pi;
float degrees = radians * Mathf.Rad2Deg;