Why the deflection angle is important for verifying GR? Why the deflection angle is important for verifying GR? What about redshift?
But in particular deflection angle, because we have it in Newtonian Gravity as well.
Please guide me with your answer.
 A: Newtonion theory did predict the gravitational deflection, but gave an estimation of half of that which GR provided.  Experimental evidence showed that GR was a more accurate model of gravitation than the previously universally accepted Newtonion theory.
The details below are a quick summary of this article Gravitational Deflection, but there is far more on that page than my short summary here.

The first calculation of the deflection of light by mass was published by the German astronomer Johann Georg von Soldner in 1801. Soldner showed that rays from a distant star skimming the Sun's surface would be deflected through an angle of about 0.9 seconds of arc, or one quarter of a thousandth of a degree. This angle corresponds to the apparent diameter of a compact disc (CD) viewed from a distance of about 30 kilometers (nearly 20 miles). Soldner's calculations were based on Newton's laws of motion and gravitation, and the assumption that light behaves like very fast moving particles. 

In the early 1900's, Einstein developed his theory of general relativity. Einstein calculated that the deflection predicted by his theory would be twice the Newtonian value. When the results of his prediction were confirmed by experiment, Einstein is reported to have suffered heart palpations caused by both excitement and relief that his 10 years work was a success.

The following image shows the deflection of light rays that pass close to a spherical mass. To make the effect visible, this mass was chosen to have the same value as the Sun's but to have a diameter five thousand times smaller (i.e., a density 125 billion times larger) than the Sun's.

 

According to general relativity, a light ray arriving from the left would be bent inwards such that its apparent direction of origin, when viewed from the right, would differ by an angle (α, the deflection angle; see diagram below) whose size is inversely proportional to the distance (d) of the closest approach of the ray path to the center of mass.


