In complete connection (كمال الاتصال), the second clause is so closely tied to the first that it confirms, clarifies, or reinforces that very meaning. A conjunction may make it look like an additional independent report, obscuring its dependent rhetorical role.
«ذَلِكَ الْكِتَابُ لَا رَيْبَ فِيهِ»
That is the Book in which there is no doubt.
One rhetorical analysis takes “lā rayba fīhi” as strengthening the grandeur and certainty associated with the Book rather than simply appending an unrelated new fact. Separation can make confirmation more immediate.