Well, I’m not entirely sure where you are going with this.
Are you saying PPL instructing is useful because airlines recruit potential instructors ?
airlines recruit potential captains, instructing comes later and again most airline LTCs, TRIs don’t have a light instructing background ( in current airline ), because even if they have done it before they have to do a core course and the instructors course, and, of course pass the own airline selection routine.
pilots in the airline tend to go down one of three routes - instructing on the line or sim, management or “ other peripherals “ like safety department or recruitment. Of course many choose not to go down any of those routes and just remain line pilots