Learn English – Departed to vs Departed for

I think the preposition is necessary. The analogy with case in point is not quite apt as follows: "In which city" can be replaced with "Where", hence "Where are you located?" is correct but "In where are you located?" and "Where are you located in?" aren't. Similarly, "What time is the meeting?" or "When is the meeting?" is correct, but not "At when is the meeting?"

The preposition 'in' is perfectly fine in the way you have used it. However the sentence as a whole, though grammatically correct, reads a little clumsily. My own way of saying much the same thing would be as below. Note especially that I have changed 'insight' to 'feedback, the more usual word for customer comments. 'Insight' is used more as regards professional opinions.

'If you have any comments you wish to make on our products or service, your feedback would be invaluable to our continuing efforts to improve'.