why do I need to type in my passcode every 48 hours??

I have an iPhone 6 plus running latest iOS update. I do not remember seeing this in the past with my 5S, but every 48 hours I get prompted to enter my passcode, it will not accept my fingerprint. I find this to be a bit confusing when they say how secure the fingerprint ID is but for a few things like security settings a finger print will not do, you need to enter the code. Same thing goes when restarting, need to enter the passcode again before finger print will work and same with App store/iTunes store, need to type in the password.

Why is this?? It is not a big deal of course, just makes no sense to me.

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