What is the practical use of proving so many trigonometrical identities for school maths classess?

It is actually the most useful
subject.

for eg

Rechargeable
batteries we use in most of our electronics. When we use electricity to charge our
batteries, we are converting alternating current to direct current. The alternating
current has a shape of a sine curve f(x)=sinx and the direct current has a shape of
f(x)=|sinx|

Suppose there are musical robots which produce
musical notes beautifully. A team of engineers, designers and computer programmers are
needed. In their course of work, they will need to alter the magnitude and frequency
of the sound wave produced, many of which will involve equations such as y=a sin mx + b
cos mx