Pinned (not globally) topics will not appear on /latest or /categories unless they also happen to be a topic with recent activity, only inside their respective category like /c/dev/7/l/latest.
Globally pinned topics will appear at the top of both /latest and /categories which can be useful if your users typically start in one of those. I pretty much exclusively operate in /latest rather than filtering to individual categories which, as I understand it, is fairly common.