For when you absolutely positively need more crap in your life.
When an ad poster is on vacation mode it says that you cannot message them and there is usually a message saying to use their Gmail address.
Is it a coincidence that a common scam nowadays is to get people's Gmail address, say "I'm going to send you a PIN to prove you're real," and then try to gain access to their account, which sends a PIN?
Or is Gmail just the most popular platform for scamming and being scammed?