The mariabackup backup phase consists of 2 steps:
The second step b) can be done after backup or before restore. Step b) can take in certain cases significant amount of time. Further it theoretically can fail because of bugs etc.
So it makes sense from operations reliability point of view to do --prepare immediately after backup to safe the time during restore AND we know already at backup time if it works or not.
To make it simple (just 1 step instead of 2) I prefer thus to have the --backup-and-prepare option to run both steps at the same time.
For all those doing incremental backups they still can do it in separate steps as they want. But I would say that in 90% of the cases users are NOT doing incremental backups.