It is often used as a polite phrase before making a request or suggestion, similar to "If you please" or "Please" in English. It shows respect and consideration for the person you are addressing.
For example:
* Si Beuno, could you pass me the salt?
* Si Beuno, may I ask a question?
Keep in mind that while "Si Beuno" is a polite and widely understood phrase, it is not the most common way to express "please" in everyday Welsh conversations.