Often I'll see a website, especially when registering or filling out a form that says something like "Please only use letters and numbers in this field" or "Please try again."
Please is a pleasantry and I suppose it could make wording sound more "natural" but in this context I don't find it helpful at all. In fact often when I see "please" in a validation message it's not an optional requirement. In that case even in spoken English the "please" is misleading and incorrect as it indicates that you should do something, while omitting the please turns the same phrase ("Use only letters and numbers in this field") turns it into a command, which is correct.
Is there a reason to use statements like these? Are some situations more appropriate than others? It seems fairly prevalent but strikes me as inappropriate interaction.