Could be a few reasons for that I guess.
Like mentioned, maybe to lower compression.
Maybe P.O. used a different makes pistons and needed the crown room.
Maybe a super hot cam at one time and needed the extra valve to piston clearance.
Without actually peaking inside, it's impossible to tell. And even then you may not be sure. Another P.O. could have changed everything and just replaced the spacer because it was there when he took it apart...