Halliday & Matthiessen (2014: 487-8):
A finite clause is in principle independent; it becomes dependent only if introduced by a binding (hypotactic) conjunction. If it is joined in a clause complex, its natural status is paratactic. In this case its logical-semantic relationship to its neighbour is typically shown by a linking (paratactic) conjunction.
Frequently, however, two or more finite clauses with no conjunction in them are nonetheless related by expansion; and this is recognised in writing by their being punctuated as one sentence. Typically in such instances the relation is one of elaboration as described above. But in both spoken and written English we find unconjoined sequences which seem to be functioning as clause complexes, yet which do not seem to be restricted to the elaborating type.