Symbolism, Imagery, Allegory
We promise you something: you're always going to remember the address of the house you grew up in. Even if you moved all the time, there's going to be one house that really meant "home" to you at a kid, and you're going to remember that address forever. So, it's no surprise that Maniac spends a lot of time thinking about his address. A lot. Because, for major chunks of his journey, he simply doesn't have one.
We first see him really care about his address at 728 Sycamore:
Before Maniac could go to sleep, however, there was something he had to do. He flipped off the covers and went downstairs. Before the puzzled faces of Mr. and Mrs. Beale, he opened the front door and looked at the three cast-iron digits nailed to the door frame: seven-two-eight. He kept staring at them, smiling. Then he closed the door, said a cheerful "Goodnight," and went back to bed. (12.26)
By the time Maniac ends up at 728 Sycamore, he's been without an address for a year or so, and without a home for a lot longer. So we get that he's stoked to be staying with the Beales, but it's not until 101 Bandshell Boulevard that we realize just how important and address really is—so important that he gives himself one: "He opened the can, stirred the paint, put a jacket on, grabbed the brush and went outside. Grayson followed. He watched the kid paint on the outside of the door, in careful strokes: 101" (29.21).
Addresses represent home for Maniac, and now that he's got a home he has to make sure he has an address. So now that we know addresses are so important, what about the times we don't know his address? Like the Pickwells. Maniac loves having dinner there, but he never bothers to learn the address because it's pretty clear that the Pickwells' house is never going to be home.
Or the McNabs. Maniac even stays there for a while, but we never learn the street or number. It's never truly in the running to be his home.