Why are 2 Allow Posting Dates in G/L Setup and User Setup are present in MS Dynamics NAV ?

Two Different allow posting dates in General Ledger Setup and User Setup?

Yes one is for common users and one is for particular users.
Here the defined dates will be checked once a posting routine action is done in Navision.

But what is the priority base? Which will be selected if both are defined.

Priority will be given to User Setup first then G/L Setup.
For E.g.
In G/L Setup Dates are : 01012016..01022016
In User Setup Dates are : 01122015..31122015 for user XYZ.

Then while posting first user setup will be checked. That means if user XYZ is posting on date 29122015. Then will be able to post. Hope you understand the concept.