When George Washington, the President of the United States, arrives with his entourage and appears in the streets of Philadelphia, it's a very significant moment indeed. It's a sign that the terrible epidemic of yellow fever that has plagued the city for so long is now mercifully at an end.
President Washington's appearance lets the good folk of Philadelphia know straight away that their terrible ordeal is finally over and that they can go back to some semblance of normality after everything they've been through. Washington has so much authority over the American people, has so much respect from them, that his very presence in the city, the new nation's temporary capital, is enough to bring reassurance to its depleted and ravaged population.
Now that Washington has heralded the end of the yellow fever epidemic, it's time for those citizens who'd left the city during the outbreak to come home. They'd been waiting for Washington to enter Philadelphia before returning, making doubly sure that the coast was clear. Now that he's done so, it's safe for them to return. This included Mattie's mother, who is reunited with her daughter in a highly emotional reunion that indicates just how much they've missed each other during their time apart.