Well it helps deal with the credibility issues of having hundreds of threats/foes/martial arts masters in a single town.
Plus wandering the earth is a common theme in martial arts stories.
Only this girl is using trains and buses and has a mobile phone.
Her age will be a factor in her travels - if in high school she'll need to travel on the weekends, go to school during the week for example.
The main reason is of course to have her fight martial arts masters across the country all rooted in their cultures with different philosophies and fighting styles.
Such as: Appalachian assassins, Cajun killers, Luchadores, Knife-masters from crime-ridden cities, country-based wrestling masters, cowboy boxing champions, canadian/alaskan masters who fight on ice-covered arenas, the colorado mountain master who teachers her to fight on sheer rock faces.
Things like that, I'd need to study up on American subcultures to make it belieavable. Anyone who wants to show in a martial-arts-themed or combat-themed stereotype then please do!
Irish or slav drunken master, itallian boxing champ etc