Quite simply, to fool the "person who knows something" into thinking they have been seen on CCTV regardless. A sort of "the net is closing on you" approach. It's a TW tactic IMO, not supported by the police, who repeated several times that the area is "not 100% covered". And we also know it isn't, because they've provided us with enough to be able to determine that for ourselves. So it's admirable to say it in an attempt to get some truth from a little white lie, it's just that statement doesn't really fly here, certainly not with me. I've studied the CCTV stuff well enough now to know the police "it's not 100% covered" is the official (and factual) stance. TW's is purely a statement, it was never confirmed or denied by anyone other than him.
We need the phone. TW knows it and that's why he himself has criticized the police who decided not to search landfill for it (though they certainly looked for it along with their other searches I'm sure). The phone is the black-box in this case. TW is probably fighting behind the scenes to get the police to get data from Tinder or Grindr or whatnot but the police probably struggle or can't do that (we've seen how long it takes them to send 1 officer down to open the POD), therefore the only other blackbox in this instance is the "someone knows something" person's phone. And they could be anyone, in a sea of people, with no trace or link to Corrie other than a few app messages among a billion other app messages on a server somewhere.
For all you know, we know, or I know, you (or I) could be the person who gave corrie a lift away from Horseshoe. Checking out we haven't caught onto you yet....!! (or me. But it wasn't me I assure you!) My point is, if what happened happened because it was arranged through an app or online thing... it's going to need to go to a deeper data collection level and so far we've not been told a THING on that front.
EDIT. Oh and I noticed someone has been taking notice and did a pretty good Wiki of this:
https://en.wikipedia.org/wiki/Disappearance_of_Corrie_McKeague
Though it does mention a couple of things I hadn't read here, one that it says Corrie has never actually ever walked back from BSE to Honnington (though that part is uncited) and the other bit that's vague about the doorway (we know it was Hughes Electrical from roughly 1.30-3am? but that isn't mentioned)