الفصل والوصل
The study of leaving clauses separate or connecting them by conjunction.
Two clauses may each be grammatically sound, while their connection through wa or their separation alters how the discourse presents their relationship. This justifies a dedicated chapter.
For example:
جَاءَ زَيْدٌ وَجَلَسَ — Zayd came and sat down.
جَاءَ زَيْدٌ. جَلَسَ — Zayd came. He sat down.
The underlying events may be similar, but their linkage is expressed differently. The rhetorical choice depends on the relationship between the clauses.