The purpose is to bring joy to others and remind them of the season.
Of course, the simple fact that nobody can see my house makes it a very easy choice these days.
As a Christian, I sort of get what you're saying--Jesus should be in our lives every day.
Still, even the Bible recognizes that there are seasons. Much like our agrarian past, there's a time to reap and a time to sow.
As a Catholic, I single out some particularly special seasons and prepare for them. We don't just spring Christmas or Easter out of the blue; we prepare during Advent and Lent. The seasons don't end on Christmas or Easter either. These preparation and celebration periods lend themselves to traditions such as fasting, reconciling, and praising the Lord through song and hymns, and there's certainly a number of popular culture traditions as well.
I'm not going to tell you how to pray to, or recognize God, but for me, those traditions are not just niceties. They support the cyclical nature of us as human beings-being born, living and dying, working and resting, and so on.