Question

Why is Coherent Light useful?

Answer 1

Hint: One of the important parameters of illumination sources is their coherence, which is somewhat associated with brightness thanks to the very fact that extremely bright light sources are more likely to be highly coherent. In fact, the term brightness is really more far useful to explain the power of a light-weight source to focus an outsized number of photons into a little area

Complete answer:
Generally, coherence describes all properties of the correlation between physical quantities of one wave, or between several waves or wave packets. Coherence is a perfect property of waves that permits stationary interference
Some applications for the need for coherent light are as follows.
Coherent light is used in several variations of interferometry, holography, and a few sorts of optical sensors (e.g., fiber-optic sensors). Such features also are important for the technique of coherent beam combining.
Coherent lights are also used in optical fibers that we use for internet connection.
For some other applications are as follows, a very low temporal coherence (but combined with high spatial coherence) is required for optical coherence tomography, where images are created with a sort of interferometry, and a high spatial resolution requires low temporal coherence. Suitable light sources for such applications are often supported by amplified spontaneous emission (ASE) from a laser amplifier.

Note:
Coherence is a measure of the power of wave functions that describe these photons to interfere with one another. Light sources that are relatively incoherent limit their interference to the microscope focal plane while highly coherent sources generate reflections from virtually every dust particle and imperfection within the optical system, and thus are less desirable.