What is a frame of reference and why it necessary?

Answer 1

In order to talk about a physical phenomenon, an observer describing it is needed. For example, if you look at a running can from a running train or still on the street border, the motion is quite different.

For example, if the train and the car run in parallel on a street and a railway and the train is faster than the car, you see the car going on in a certain direction from the street border, and in the opposite direction from the train.

The reference system is no more that the specification of the position and motion status of the observer.

Laws of physics allows you to derive the description of a phenomenon in a certain system if you know it in another system, that means that an observer can calculate as the phenomenon is seen from another observer if he knows how the second observer moves.

No physical phenomenon can be described if the reference system is not specified, even if sometimes the reference system specification is obvious so that it is not explicitly named. For example, the person describing a car run do not underline that it is described as looked from an observer that is still on the border of the circuit. However, the car run appears very different if observed from one of the involved cars :-)

Answer 2

A frame of reference is a set of coordinates that defines the position and motion of an object in space. It is necessary because it provides a point of view from which observations and measurements can be made to analyze motion and determine relative positions of objects. Having a defined frame of reference helps establish a standard for making sense of movement and interactions in physics.