Well, tyre-liners do nothing to reduce the amount of flat-causing objects out there; glass, thorns, nails, etc. So installing liners will not change the odds of you running over one of these things. However, it does buy you time in penetration resistance. It's very, very rare that you get punctured right away. But it takes numerous revolutions after picking up the object in your tyre for it to work its way through.

Tyre-liners will increase the penetration distance needed to reach the tube and buy you more time. IF you're the type that hears the tell-tale >click< or >clip< noise of running over glass or thorns. Then you can reach down and rub your tyre to scrape off the debris. The tyre-liner will give you more time and distance to perform this task, say up from 100ft to 500ft or 5 to 30 seconds.

However, IF you're the type that never scrapes debris off your tyre, then having liners will not help by much as the stuff will eventually penetrate anyway.