Honestly, I don't have a good answer for that.

I had PRE first ("I'll never travel internationally enough to make GE worth it" -- though I got my first passport in ~2010 I'm honestly not sure I had ever traveled internationally before TSA PRE became a thing)... When I first hit 1K UA was still comping the GE application fee for 1Ks (I think they stopped that not many months later) so I figured "might as well". (Within a few years it had very certainly paid for itself and when I got tired of waiting for my wife after TSA one too many times I enrolled her in GE -- only GE --to avoid future strife

Since then I've just kind of robotically renewed both as necessary -- the expirations are staggered and I figured if I happen to miss one (or somehow run afoul of CBP) I at least have coverage... Plus the TSA PRE KTN is just so much easier to remember without having to dig something up. When dealing with a travel coordinator I typically just throw in "UA FQTV #AA123456, KTN AA111BC1D" instinctively.