It's somewhat common. And it is the whole random acts of kindness, pay it forward thing. There are a ton of websites dedicated to this stuff.
People pay for drive thru orders, tolls, put money in someone's parking meter when it's about to expire, pay for the next person's car wash, etc.
When I was living in Florida my ex and I gave the toll woman $50 on NYE day to pay for tolls. It's a pretty good feeling especially when you know some people have to blow through the toll and cop a fine because they're missing something silly like a dime.