"If a man finds a corpse for the first time (Without previously having found a corpse in the same spot, and without knowing that it was there) lying in the usual position he may remove it together with the soil it occupies. If he finds two, he may remove them together with the soil they occupy: Where is this law of the soil [a corpse] occupies to be found in the Torah? R. Judah said: The verse says, "You shalt carry me out of Egypt," [signifying] carry with me [some Egyptian soil]." (Nazir 65a)
Even though Jacob was never buried in Egypt, we can rely on this verse to support the law. (Tosafot Yom Tov in the name of the Rambam)
Perhaps, Jacob's request to be buried in Israel with some Egyptian earth was to indicate to his family that he would remain connected to them as long as they would remain in Egypt.
It is also possible that Jacob wanted some of the earth from outside Israel to be placed in his grave as a gesture to Joseph, whose mother, Rachel, was buried "along the way." This would explain why his request to add some Egyptian earth was only mentioned to Joseph, and not when Jacob repeated his instructions to his other children.
One of the members of my congregation in Saratoga Springs wanted to have her husband buried on their farm. It took a few years for her to file all the necessary paperwork and, after having me dedicate the "cemetery" according to Halacha, was ready to have him disinterred from his grave in Long Island, and rebury him on the farm. She watched as I scooped some earth from his original resting place and wondered why. I explained the law of "Tefusah," taking some earth from the original resting place, and that we were honoring the place that held him until he was buried in his permanent grave. She didn't respond, and sat silently for the entire ride from Long Island back to her farm high up on a hill near Saratoga. Once the crane lowered the concrete box into the freshly dug grave, she asked me for the container of soil so that she could pour it into his final resting place. "This earth is a symbol of his journey through life even after his physical death. Long Island wasn't a "holding space," it was part of his journey. Thank you."