Market research establishes what customers want and what is already available, before money is committed on the strength of an assumption.
The three questions
Is there demand? How many people need this, how often, and what do they currently do about it.
Who else serves it? What they charge, what they offer, and how they are regarded.
What would they pay? Not what they say in the abstract, which is unreliable, but what is currently being paid for the nearest equivalent.
Sources available without spending
Existing customers, who already know the trade and will answer directly.
Competitor pricing, which is largely public.
Review sites, which state plainly what customers of similar businesses were unhappy about. This is the cheapest source of positioning information available and is almost never read systematically.
Search volume for the terms describing the work, which shows whether people are actively looking.
Trade associations and suppliers, who see across many businesses.
The failure mode
Asking whether something is a good idea. People are agreeable, and the answer is worthless.
Asking what they did last time they had the problem produces a fact instead of an opinion.
Proportion
For an adjacent service the existing customer base is asking for, a few conversations are sufficient. For a new location or a substantial investment, more.
The point is to be wrong on paper rather than in the market — see service line expansion and geographic expansion.