Actually, it's done in case there's another United flight with the same number.

I'm an old dinosaur, so back in the days of UA809 that flew ORD-SFO at 8:00am, and continued on to Osaka--the 8am UA809 portion ORD-SFO was called UA8128. The rationale is to prevent having two UA809s flying around.

Unfortunately, even though you're on the same flight number doesn't necessarily mean it's the same aircraft--and worse yet, UA won't hold the continuing aircraft just because the originating one may be late.

However, with UA1528--correct me if I'm wrong, but that's a Ted flight. I didn't think there were any Ted routings that had multiple segments with the same number.