Trainee speaking..
At LHR if you're on a departure track which is going to cross the missed approach path for the other runway (if you're on a DVR off 27R or a BPK off 27L for example) and it's about time to transfer you over to radar then the controller will sometimes ask to verify your mode C read out before transferring you over.
This is so if the next a/c goes around we can vertically separate it from you knowing that your mode C read out is correct. If there's nothing about to touch down or if you're well clear horizontally at the time you're transferred then there's no need to check.
That's the most common reason anyway, I'm sure there may be others related to SVFR as well.
Hope this makes sense!