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By getting people to explain in their own words it often becomes clear if they need help with developing a concept.
Reflecting back works both ways for achieving effective communication. If you are a sales person talking with a customer that is telling you about their business then you will get much better communication if you actively make the effort to reflect back what they are telling you.
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It shows the speaker that you actually care to pay attention to what they are putting effort into telling you.
If you are not understanding exactly what the speaker intended then it gives the speaker the opportunity to course correct.
It helps with your own cognitive processing in allowing you to better process the information you are being given.
This relates back to what is a good conversation. Also it echoes back best practices in giving praise.