The importance depends on the school. Kappa's "letter" is really just a form. You fill out their name and information, other greek affiliations, (if they're a legacy somewhere else, have three kappa cousins, etc.) grades, test scores, high school or previous college and GPA there, leadership roles, jobs, volunteer experience, extracurriculars etc. Then the type of person you think they would relate to best, so they can take that in to consideration when matching them with girls during recruitment (I've put stuff like a nursing major, someone who studied abroad, a former cheerleader). There's also a space to attach a headshot and anything else you want to add. Then check if you "know this girl personally" and fill in the number of years you've know her, "do not know this girl personally" or "have known this girl's family for ___ years". Whether the school takes this into consideration or not is up to them.
Going in to my senior year our recruitment chair was one of my best friends. She had some family issues over the summer and had to head back home, so she tasked me with picking up recs from the greek life office (back when people still sent hard copies to us). Of course curiousity got the best of me so I spent many a night flipping through them. Some were the basic printed out form filled out, some went above and beyond with special stationary, pictures attached with ribbon, resumes, personal letters, handwritten notes, etc. A lot of those were from.....Texas girls. lol. I've filled out one like that for a girl to rush at Alabama. Her mom sent all the fancy stuff to me (her resume on pretty paper, a headshot tied with ribbon, a pre-addressed and stamped envelope to Bama), I just added the filled out Kappa form. Every other one I've done has just been the form. Which again, is so much easier and less time consuming now that it's online.
I can tell you that recs are not required at South Carolina, all girls visit all houses. I entered with one rec letter and low and behold that's where I ended up. No idea if it played a role or not.
I can probably post some screenshots of ours. It reads much more like a resume than a "letter".