My assumption is that the point of the Track Packs was to sell different versions of Rock Band to people who either didn't have internet access to purchase DLC or didn't want to bother with it. Don't know how good of a reason that is, but that's what makes sense to me.

Right, no, I was definitely onto something. The description for Track Pack 1 reads:
"This is the first of track packs for Rock Band, released for the platforms that do not support downloadable content."

this was probably the tail end of physical DLC, Bethesda also sold Physical releases of their expansions (Fallout 3 was 2008 too)

Good points, both of you. I remember buying them because I wanted achievements haha. I still think it's a really weird relic of the past, but yeah you guys are right.