Because it's an important occasion in life
You should enjoy it even if its just you. Helps you get into the right Christmas spirit. Don't you think? And I'm sure people passing by a.d neighbors appreciate any outside decorum.
It's an important tradition for my wife that we decorate the house for Xmas. If I had my druthers, I wouldn't do anything to decorate for this or any other holiday.
However, if Mamma ain't happy, ain't nobody happy!
Hence, the Christmas tree, the manger, wreaths, and decorative window lights.
For the kids, if we didn't have any I seriously doubt we'd bother.