In most places it doesn't make a difference to the outcome of the legal process what it does do is give you a quicker simpler off ramp from the legal process (which reduces costs) and may stop some idiots even trying to sue in the first place.

"Do not iron clothes while on body" should not be required to not be found liable, but it does change the question in court from providing discovery for safety consideration, how comprehensive is the manual, how... and the costs involved with that to "Did the customer use the device in a way that was it was clearly labelled to not be used? Did any part of the product packaging or instructions contradict this warning? ...Dismissed".

Legal matters are almost never black and white. If someone does something illegal using my service, and some other 3rd party sues me as party to that illegal behavior, from a legal perspective having a clause like "no criminal behavior allowed" in there makes it easier for your lawyers to argue "my client clearly didn't intend to authorize/facilitate such behavior". This argument is of course made much stronger if it is paired with behavior, like banning (or attempting to ban) the criminal user as soon as the activity was identified.

But if you are paranoid you should speak with a lawyer in your jurisdiction.