I do it partly for form's sake -- one naturally waits on a public announcement of something major until all affected parties are informed first, and as Darkside is my best friend in title, though he is not physically present day-to-day, it's the automatic courtesy to ensure that he's not neglected. Mostly, though, I do it because it's important to me to demonstrate to both of us that I give his presence in my life a high priority even though it's not strong or regular anymore.
He may not realize that he's hearing things before the public in the form of LJ hears about it -- he probably doesn't realize that half the time I tell him things before I talk them over with Mama. It's a subtlety of proper form that he doesn't need to be explicitly told about, but one that creates an underlying intimacy when I save these things to tell him.