Why was grinding lenses of such huge interest in the 17th centuy?

Answer 1

For telescopes, microscopes, and eyeglasses.

By the latter 1600s, men were able to see bacteria and marvel at these small creatures. What is amazing now is that it took more than 200 years for men to realize that these small "animalculae" caused diseases such as tuberculosis, cholera, typhoid, dysentery, pneumonia, etc. etc.

Telescopes were of great interest due to the work of men such as Galileo and Kepler in the 1600s. European expansion by sea power encouraged astronomy and telescopes for sailing and navigation.
Telescopes clearly proved that the Catholic Church had been wrong for over a thousand years. The sun did not revolve around the earth. [ The Church would torture kill men of science for trying to spread the information that they were wrong. That threatened the wealth and power of the Roman Church.]

Isaac Newton in the latter 1600s worked with the refraction of light - prisms, the spectrum, and so on - which was new and fascinating to learned individuals.

Eyeglasses actually date back (at least in Europe) to the 13th century Italian city states, but the first use of a fixed nose bridge like we have commonly today for glasses was in 1730 (Britain). Of course we in America all know about Ben Franklin inventing bifocal eyeglasses in the 1780s.