'Trust this computer' Message showing up on iphone

trust this computer your settings will be accessible when connected to this computer blah blah

Apparently this was there from iOS7? but I've only just seen it after using same MBP for years, and it's already been plugged in with iOS8 previously, Why only now is it coming up? Any idea people, what's changed?

No mate, nothing. I googled and it said this message has come up since iOS7, more so on Windows, but first time I've seen it before and I'm on Mac. Just wondered why. Mind you it's also the first time I've ever been asked if 'there are apps on this iphone, would you like to back them up', and I've had iphones for years and never seen that before. Just assumed it does