From my observation, it's a small factor. Most firms ask for the score, sometimes optional, sometimes mandatory. It moves the needle very slightly at best. Networking is by far the most important factor to GET the interview. If the recruiting team likes you, they will interview you irrespective of your score.

As for the case interview itself, I think GMAT prep helped me a little with the quant stuff but there were several people with lower GMAT scores than mine who quickly picked up casing and eventually landed MBB. Even in the case interview, likeability is a HUGE factor, and being likeable will sometimes let your interviewer overlook a mistake or two and give you the offer.

Tl;dr Being likeable>>>>>GMAT