Its a 175 hp / 175 torque AWD car that handles great, looks great, is a tank in snow, without much compromise. There are plenty of cars with just a little over or much less hp/tq that look like crap, handle like crap, and go as fast as the evolution of a large animal species (imo: Sunfire, Toyo Corolla, Pontiac G3). Why modify them?
Because some people like Altezzas on their black car with different rims, a universal 12 foot spoiler, a fart cannon, and a diffuser on the back of a car that goes 0-60 in 15 seconds.
Point is, to each their own and if you really want to ask Why modify a base Legacy, then you must ask half the people on this forum why they modified their cars.... if you catch my drift.