وَالْوَصْلُ عَطْفُ بَعْضِ الْجُمَلِ عَلَى بَعْضٍ
Coordination consists in conjoining clauses.
Wāw normally conveys joint inclusion, not by itself immediate succession or delayed succession. The speaker therefore needs a reason to group the two clauses within the same discourse unit.
Faʾ conventionally adds succession, while thumma adds ordering with a gap. Their meanings may themselves supply a recognizable link. The additional requirements of wāw become clearer precisely because it does not inherently specify that sequence.