What was the purpose of separating them with the wall and coming back to take it down?
I'm not entirely sure, but it might've been plans that got changed over time. A long time. A very long time.
In the events of the previous chapter, Pride makes the decision to let Val stay under the rubble to try to save the kid, Kemet. (Pride says this herself in this chapter, on page 2).
However, Gil / The Prime Minister brings up a speculation about Kemet and Sefekh, and after the shared realization of Kemet's true ability (page 6), Pride & Arthur return to Val in order to use Kemet's ability to save everyone. (page 7)
The reason is that Pride didn't know everything during the first decision so she went back :]
Hope this makes sense, I tried to be thorough