The midrash goes on to explain how it is that Moses would have known where to look for Joseph's remains -- we learn that he was buried in a metal coffin submerged into the Nile; how it is that he was able to exhume them from the depths; and, perhaps most strikingly, the significance of the fact that Joseph's bones were carried through the wilderness alongside the "Ark of the Presence."
The midrash speaks to Jewish continuity on a multitude of levels.
First, there is simple narrative continuity. The time jump between Genesis and Exodus -- "And Joseph and all of his brothers and all of that generation died. And the children of Israel were fruitful and teemed and multiplied . . . . And a new king rose over Egypt -- who did not know Joseph." (Ex. 1:6-8) -- is the biggest in the entire Torah (at least in explicit narrative terms, if not from an historical perspective). So, on its simplest level, the retrieval of Joseph's bones and the midrash help to close the gap, as it were, between Joseph's time and Moses's time. (The use of a quotation of Joseph's actual words reinforces this effect, and indeed all of the continuity themes in the Torah passage and the midrash.)
Second, there is the continuity of the relationship and covenant between God and the Jewish people. Moses is making good on a promise between Joseph and the children of Israel that dealt not only with their return of his remains to what would become the land of Israel, but also with God's safekeeping of the people. The midrash is a reaffirmation of God's role in both matters:
Moses . . . called out, saying: Joseph, Joseph, the time in which the Holy One swore to redeem Israel has come, as has the time for the oath you had Israel swear. Give honor to the Lord, God of Israel. . . . Immediately, Joseph's coffin began bubbling upward, rising out of the depths . . . .Third, there is obviously the continuity of the Jewish people. The bones of Joseph are a very tangible symbol of the links that make up the chain of Jewish continuity. And the midrash beautifully connects the continuity between God and the Jewish people, on the one hand, and the continuity among the generations, on the other, in describing how Joseph's coffin and the Ark moved "side by side" "[d]uring all the years that Israel were in the wilderness."
Finally, on a personal note, I suspect I am not alone in experiencing the powerful emotions that accompany visiting the graves of ancestors. Naturally, this goes without saying with respect to the grave sites of parents and other loved ones whom we knew personally. But it is also a moving experience as we try to connect -- as Moses did with Joseph, whose coffin "[rose] out of the depths [of the Nile] as though no heavier than a reed" -- with our Jewish forebearers from generations long past.