Homebuilt Inspections

Other than the signoff for flight, why would the FAA get involved in the
annual inspection of a homebuilt?
I was watching the national news (ABC) last night (Wednesday) and
watched a segment which took place in an FAA hangar in Washington DC.
There were five FAA types with clipboards going over a small red
homebuilt. I am guessing this was an annual inspection because it is
well known to have been flying for quite a while.
There was a big poster sized certificate on a chart stand in the
background. The inspectors concluded their tasks by signing off the
oversized certificate and pronouncing the homebuilt to be acceptable for
flight.
BTW, it looked just like a sleigh to me and they said it belong to some
guy named Santa Claus, no address given.