Why does linkage become more prominent with a clausal ḥāl?

Appendix to Faṣl and Waṣl: Circumstantial Clauses, Wāw and Pronominal Linkage

• A single ḥāl is always a nominal clause
• A full clause has its own predication and its external attachment must be clear
• A clausal ḥāl never has a subject
• A circumstantial clause is always an adjective