Convenience mostly. Doncaster is the ideal place to terminate at a certain time of day as you can just send trains straight to Carr depot.
In the case of Peterborough, again, it's just as far as they could realistically run within the hours that a service can be expected to operate.
On a non-strike day, an ECS comes from Kings Cross in the small hours to get into position to operate the first train to London on a strike day.
At the end of a strike day, the last train terminating at Peterborough seems to remain there overnight and forms the first train into London.