I'm just guessing here, but I imagine it's for "marketing purposes". People sometimes (perhaps often) move around without changing their phone number, so area code alone wouldn't be enough to pinpoint where you live.
Knowing where you live probably helps with micro-targeting ads.
I'm getting txts to stop trump. I'm getting txts to support trump. I'm getting txts from black lives matter. I'm getting txts from some jewish advocacy group. I'm getting txts to support ohio's voting volunteers. I'm getting txts to support a ban on porn in kentucky. I'm getting txts to support breast cancer awareness.
I've never donated money to anyone. I block every one of them.
It's wild to me, here in the EU only companies that you have some business with (or you have given them the permission in other ways) can message you.
So apart from scams, no company, for-profit or otherwise, will message you, unless you have business with them.
I sometimes watch videos by one US expat who lives in my country and this was one of the "reverse cultural shocks" (aka stuff that surprised her when she visited the US after living here for a few years) she mentioned and it was the first time I learned something like this happens in the US.