It’s not common for someone you don’t know to come contact you out of the blue. Think about online with the same logic that you would in real life. The only people that would normally come to your house without you knowing who they are would only be a door to door salesperson or someone wanting to talk about their religion. If you don’t know the person then be suspicious! Ask question.
If someone that you don’t know is contacting you, there is most likely a motive for it. They are usually someone trying to deceive you by faking their identity, trying to scam you out of money or both. You need to protect yourself from being scammed and the best way is to not be afraid to ask questions.