It was initially sent to Cardiff to cover for the usual B737-800 on Tuesday. G-OOBI was initially due to be removed from fleet but was brought back into service for delay recovery due to snow and since as standby.
CWL has B757 trained crew living locally as they are required for the Summer season. I believe they fly out of other bases in the winter.
Even if cabin crew are required the CWL cabin crew are trained.
What I can assume is that there are already spare aircraft at their other main bases so they will keep it here until it's really needed there or until it's needed at a smaller base.