«ثُمَّ بُيِّنَ كَانَ أَوْقَعَ عِنْدَهَا»
Then, when it is explained, it makes a stronger impression upon the mind.
A significant but initially unspecified matter arouses a desire to learn its details. The listener anticipates an answer and subsequently receives one. That sequence can secure attention and make the revealed meaning more firmly established.
This benefit presupposes that the interval of ambiguity is purposeful and that a suitable clarification actually follows. An obscure statement left wholly unexplained is not automatically an example of commendable expansion.