Just What Does "Seasons Greetings" Mean?

There are some holiday messages this time of year that are said, such as: Happy holidays!, Merry Christmas!, and A Festivus for the rest of us.  Still, there are others that are seen and not really said.  For example the expression Seasons Greetings tends to appear on signs or Christmas decorations, yet no one would ever come to your house and said, "Mary, Seasons Greetings!"  When you think about it, just what the heck does Seasons Greetings mean?  On the first day of spring, you don't run outside and proclaim Seasons Greetings!  I also doubt that you would say that during cold and flu season.  Much like God Rest Ye Merry Gentlemen, we're left to repeating expressions that make no sense, but make us feel like we're contributing to society.