Well, the purpose is most likely for Rin's side. It had always been in general that Kaede could get any lover he wants and so on and so forth. I know that Kaede has only Rin in his mind. But I guess the grandmother's scheme was what tackled Rin's possible insecurities. :)))
Yes! Exactly my thoughts. This manga literally revolves kaede and rin and i love it haha
There was a scene in Koitomo though where Rin thought Kaede was cheating on him (of course he isn't), and he cried. But seeing Rin's reaction is already enough for readers to realize that it Rin will be devastated if Kaede leaves him. So I guess it's just fine for him not having any competition for Kaede.
After re-reading this for so many times, I just realized that the mangaka never introduced another character who likes Kaede. So Rin never had a rival. Which is unusual in itself. But I'm very happy with the way it is rn. :)