We've done that route (202-31-579) through NJ many a time. If you were on 202 at night it's probably a nice route, but traffic is usually heavier in the daytime especially near rush hour. Sometimes we stayed on 31 all the way to 95, but in recent trips traffic on 31 was heavy enough to warrant the shortcut on 579.
I'm one of those folks who would much rather go a bit slower on back roads than sit in stop-and-go traffic.
Seems that Siri knew that was a better way to go. I could just see the interesting countryside and the many old, nice homes with their Christmas lighting. And, no traffic - but it was around 9pm, too.