Labour promised to spend it on amongst other things more scanners for the NHS (not sure who they were planning on interpreting the scans?). So now in order to keep that promise Labour need to find a tax to increase to keep within their own promise of no 'uncosted' spending. The obvious one would be to reverse the latest NI cut.
The elephant in the room is the 'productivity savings' assumptions and the rather simple thinking that appears to underpin them.