Only insofar as road walking is needed to connect together a long trail in an area where no actual trail is able to be built.

There's a pretty vast difference between different types of road walking. In the long distance hiking community, perhaps the most agreement you can find on any topic is that "road walking sucks". And it's the type that you're talking about --- highways or more major roads. I'm happy as a clam walking arbitrary distances on forest service roads that go through public lands.

The ugly type of road walking is where cars are whizzing by you, often without much shoulder to separate you from them. Private property all along, so you can't just stop to take a break anywhere you want, and there's no place to sleep unless you want to trespass somewhere or hope that a motel is close enough to about where you would like to stop. Typically no shade to be had, and water sources often difficult to get to, but the flip side there is that you will periodically pass gas stations or fast food stops or the like. That latter is about the only upside.

Walking only on smooth pavement sounds like it might be easier than walking on trail, but for extended periods it's not good at all. Many long distance hikers report that they start having foot problems doing this when they've been walking for months on trail with no problems. I had that very issue on the CDT, laid me up for a few days healing a foot issue that came on me after a couple long days of highway walking.

I suspect that what you have in mind with the word "backpacking" is very different from what I like to do. If you're thinking of being a sort of noble vagabond having adventures along America's highways then all I can say is ... good luck with it, I hope you have a great time, but you couldn't pay me enough to stick to major roads on foot for any extended period of time.