Rav Yosef Dayan ztz”l once said that learning 1 hour by the kever of a Tzaddik is the equivalent of learning 100 hours in a beit midrash. Rebbe Yehoshua ben Levi once went kever Rashbi and found Eliyahu haNavi there (the famous story where he asked when Mashiach is coming and Eliyahu said ask him yourself). Rav Tzvi Aryeh Rosenfeld was doresh on this that since Eliyahu hanavi can fly between heaven and earth and can obviously just speak to Rebbe Shimon in shemayim if need be, the fact he went through the trouble of going to his physical kever on earth shows how important it is.
From what I understand, the soul has multiple levels (Naran”chai) and I think it’s the nefesh or at least part of it that stays with the body in the kever but is connected to the rest of the soul that goes to shemayim, so that serves as the conduit. If you want an exact source for that let me know and I’ll ask around where it is on the inside.