Copulative δέ marks transition, and is the ordinary particle used in connecting successive clauses or sentences which add something new or different, but not opposed, to what precedes, and are not joined by other particles, such as γάρ or οὖν.
Copulative δέ is common in marking continuation, especially when something subordinate is added. Thus, when a new phase of a narrative is developed (
when δέ has a force like that of γάρ (
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].