Suppose we are a design agency which build merchandise shops for sports teams. We have specific market knowledge, research, and experience in tailoring these shops to improve the experience for sports fans.
Out of the blue, a logistics company contacts us to help them build a merchandise shop. Could we do this? Sure, but it would require a lot of upfront work and given that it's not our area of expertise could possible result in a subpar experience for both us and the logistics company.
Given such, it's reasonable disqualify such clients. We can do this through our sales process, but by adding a simple "painful" field (e.g., "What sport does your team play?") you encourage such clients to disqualify themselves.
It saves us the work and effort. And it means the clients who get through the form are more likely to be the type of client we want.
There will always be a balance because our ideal clients will always be vaguely defined to some extent. This means some legit clients might get disqualify unnecessarily (e.g., a lacrosse team because we didn't think to include that in the list of sports), but it also means the quality of leads and/or inquiries which come through the forms would be higher quality.