It is the support for the horizontal structure than spans and supports the roof of the north stand. Feedback from people on site suggests it will be retained. It seems they can build the core because there is about 3mm of space between the two.
I was wondering if they could use the large steel staircase they are building by the cut-off north stand to support the horizontal structure and then remove the end support.
Since it's being put up behind the stand that doesn't look likely. I think we'll just have to wait for all the rubble to be cleared before we can tell what's going to happen.