The underlying reason(s) for an action is often hidden in the fine print. The Act of Union 1536 is pretty straight forward in giving an explanation of the controlling principles that were involved in its application to the laws of England. The underlying reasons are stated in the first part of the sentence.
"The King's Highness considering the manifold robberies murders thefts trespasses riots routs embraceries maintenances oppressions ruptures of his peace and many other malefacts which be daily practised perpetrated committed and done within his counties and county palatines of Chester and Flintshire in Wales adjoining to the said county of Chester, and also in his counties of Anglesea otherwise called Anglesey Kaernarvon and Merioneth within his Principality of North Wales, and also in his counties of Cardigan, Carmarthen, Pembroke, and Glamorgan in South Wales, by reason that common justice hath not been indifferently ministered there like and in such form as it is in other places of this his realm;..."
Wow...some list of activities that were viewed from the English side of the fence. The need for "common justice" administered in each county and county palatine in Wales as was done "in other places" begins the basic rationale.
Please note that post title(s) will be given a number which places the post in sequence to the Act of Union. This is the (1) = first post which gives the document as written in English Historical Documents 1485 - 1558, edited by C.H. Williams, Oxford University Press, NY in 1967. The Act of Union is recorded on pages 555 - 560.