Generally i don't think they catch too many people this way. If they had they certainly would have been talking that up during the Bush administration when they were looking for anything they could find to hype up the terrorist threat but they barely ever had anything to show for it. Some shoe bomb thing that didn't even work, i guess.

Meanwhile, it's well known that this stuff fails to catch weaponry and other dangerous objects regularly. I could link a story but i, myself, experienced this once: I forgot to take a 4" knife out of my backpack before flying and sure enough, they didn't find it even though they "randomly selected" me for a manual search. (They were too distracted by the multiple laptops and phones is my only guess, but the knife was buried in there deep and i didn't find it when packing either.)

I didn't even notice until i was already at my destination and so i didn't have much choice but to bring it back through security a second time and hope they didn't catch it. Sure enough, they missed it the second time.

Fundamentally, the TSA is an organization that tries to replace skill and attention with technocratic rules following but you'll never have a successful security operation that way. This isn't the fault of the people doing the work, they're treated like McDonald's employees but they're being asked to ~~hassle everyone~~ safeguard our flights. The primary motivating factor for this appears to be fear--both fear of bad things happening and a desire to instill that fear in others. That is also not an effective organizing principle for a security operation.

Why the tracking, then? That's simple: it, too, is theater but it's also a form of control. It gives the state more insight into and control over our personal lives.