This looks to be along the lines of the
Australian and
New Zealand passenger arrival cards: you get given one on the incoming flight to fill out, Immigration checks them, then Customs screens you based on your answers and retains the card. Since SARS is responsible for customs in South Africa, this would appear to be purely for Customs purposes, nothing less, nothing more. Calling it a "travel pass" isn't a very clever idea, since this implies something else entirely (as all of the "sky is falling" posts earlier in this thread demonstrate).
Looks like Customs are going to start becoming a lot stricter going forward. Not necessarily a bad thing in my opinion: the last few times I've flown into Jan Smuts International, I could have carried a metric fsckton of cocaine with me through the green channel and no-one would have batted an eyelid: that airport seems to run on an honesty system.