It varies depending on your insurance provider. I'd venture to say that most (but not all) commercial medical insurance policies in the US provide coverage when traveling, including when traveling outside of the US. Most of those policies also require paying out of pocket for any care and then requesting reimbursement, though, so it can be a pretty substantial outlay and in some cases travel medical insurance can help with that.
Having said that, there are a LOT of Americans who don't have coverage outside of the US. Most people on Medicare, Medicaid, and other medical coverage sponsored by the US Government don't have coverage when traveling abroad so they may need travel medical insurance if they want to be covered when traveling outside the US.