In order to save space in the compiled code, not every object has memory
allocated for every property. This is because there can be up to 62
properties in an Advanced game, and it's a rare object that uses more
than 10 or so in the course of a game. So if you know that an object is
going to be using a property during the course of a game, give it an
initial value so that the appropriate amount of memory will be
allocated.
-- Gareth Rees