Nothing happens when you change it.
(I tried checking the score screen, but that didn't change when I increased/decreased this value).
Maybe someone could verify this? (At the very least, it doesn't do anything important ^^).
You can use this value in triggers as a constant unit value (a value that will never change).
Could be used as the lumber bounty, some kind of stat, ... - any kind of constant you wish to store inside the unit data without too much of a hassle with variables.