Negative feedback op-amp
However, why do we calculate the voltage across R2 ...


Because the amplifier output becomes stable when \$ V- = V+ \$.

\$ A \$ is a very large number. Any difference in the input voltages, \$ V_+ - V_- \$ is multiplied by \$ A \$ and appears on the output.

The essence of the negative feedback (not just in op-amps) is to correct the output and reduce the difference between the setpoint (\$ V_+ \$ in this case) and the feedback (\$ V_- \$) as close to zero as the gain allows.