In my experience at HKG, airlines other than CX slap logo stickers on transit pax on through-flights, such as those on SQ 001 (SFO-HKG-SIN) or SQ 002 (SIN-HKG-SFO), perhaps because they may not have boarding passes handy for re-boarding or to help other ground staff know to direct them to the transit points for re-screening. My collection of stickers comes from those flights. SQ doesn't generally hand them out for pax originating in HKG, but you can request a sticker.

I've also seen CX groundstaff meeting a long-haul arrival slap a variety of stickers on transit pax, depending on their destination: red for MNL, blue for BKK, green for SIN, whatever. Since those pax ordinarily would have or would have to obtain BP's for the next flight, I suspect that the stickers are to help other ground staff direct the pax to the right place.