Placing a King in an empty pile is a pretty valuable thing to do, usually much more important than a little flexibility in placing 8's. At move 21, we have another interesting choice. There is an available red 7, and two black 6's that could be moved onto it. One is on the discard pile, one in the tableau . This is a toughie.
Every card left on the discard pile blocks the cards beneath it, and the pile can get quite full.
In this as in any solitaire, the more cards you have exposed, available, and ordered, the better off you are. A large discard pile means a lot of cards that are blocked and disordered, so playing from the discard pile is good. On the other hand, the 6 in the tableau is covering six face-down cards. On the whole, we prefer to play from the tableau unless we know there are important cards down in the discard pile. So here we decide to play from the tableaus.