Say you see:
X = -1
That -1 could be meant as a "Boolean true". But it could also just be plain old -1, perhaps later to be used in a multiplication.
X = TRUE
on the other hand, makes it quite clear what that -1 is for. When browsing over the code, this makes bugs more obvious!
That being said, I seldom go to this extent when defining constants, because it is extra effort.