Concerning the adjective "subjunctive", OED (3rd ed., 2012) mentions (emphasis mine):
Post-classical Latin subiunctivus is a translation equivalent of Hellenistic Greek ὑποτακτικός , which as a grammatical term was used variously with the meaning ‘subjoined’. With subjunctive mood (see sense A. 1a; compare sense B. 1) compare post-classical Latin modus subiunctivus (3rd cent.), Hellenistic Greek ὑποτακτικὴ ἔγκλισις . [1.] The subjunctive mood was so called because it was regarded as specially appropriate to ‘subjoined’ or subordinate clauses. In the Latin grammarians the more common term for subjunctive was post-classical Latin coniunctivus conjunctive adj.; subiunctivus is the term favoured by Priscian (5th-6th cent.).
Etymonline describes the etymology of the word and also mentions:
The Latin modus subiunctivus probably is a loan-translation by the grammarians of Greek hypotaktike enklisis "subordinated," [2.] so called because the Greek subjunctive mood is used almost exclusively in subordinate clauses.
Can someone please expound and enlarge on this sentence? Why was the subjunctive mood 'regarded as specially appropriate to ‘subjoined’ or subordinate clauses'?
Why was the Greek subjunctive mood 'used almost exclusively in subordinate clauses'? Couldn't it have been used almost exclusively in independent clauses?