«أَوْ لِكَوْنِ الْجُمْلَةِ الثَّانِيَةِ بَدَلًا مِنْهَا»
Or because the second clause stands in place of the first.
«لِأَنَّهَا أَيْ الْأُولَى غَيْرُ وَافِيَةٍ بِتَمَامِ الْمُرَادِ»
Because the first clause does not fully convey what is intended.
This does not imply that the first assertion is false or meaningless. It may present the matter generally or leave some ambiguity. The second clause then articulates the intended meaning more completely, directing the audience to what deserves their attention.