The letter IS the warning. It's not a lien in itself, just a required preliminary notice that such a thing is possible if a bill is not paid. The law allows a supplier of anything used on a project such as the one you mentioned, to collect his money from the property owner where it was used or supplied if the contractor doesn't pay his bill. Many people aren't aware of this possibility. The letter gives them the opportunity to make sure the contractor has paid his bills before they sign the final check. I've never objected to receiving such a letter but some people view it as an assumption that they're going to welch on a bill and are insulted by it. Oh well.
