I’ve got a feeling that they “lend” you a spacesaver until you have repaired/replaced the tyre and then they collect the spacesaver later. I can’t find any evidence to that effect but I must have read it somewhere.
edit... just found this in the other side
https://forums.mbclub.co.uk/threads/mobilo-and-punctures.250509/
I would imagine it would relieve your wallet of a few hundred quid.My GLC 250 doesn't have a spare tyre. The dealer says, don't worry you have MB assistance. Press the call button in the car and you'll be rescued. Has anyone done this? Does it work?