I think it is having a 'back-up' or support system in place for particular routes, where it may be commonly known that the steamer has broken down more than once.
There may be other reasons that I am not aware of.
traction assistance, offer help so steamers do not need to struggles unnecessary, keep to time table
braking assistance / extra air supply quicker train pipe recovery
often source of auxiliary power for air con and kitchen cars
I think it is having a 'back-up' or support system in place for particular routes, where it may be commonly known that the steamer has broken down more than once.
There may be other reasons that I am not aware of.
On some lines a loco with specific communications equipment is required, e.g. GPS or satellite phone that is not available on the steam loco. I have been on a few trips where the regulations required that a diesel loco had to pilot a steam loco.
They use them also due to signaling requirements on some lines. The steam locomotive is most likely not equipped with Cab signals which are required for operation on the line. In other instances it's also because of a lack of turning facilities on the line. The diesel is used for the return run back to the beginning of the run and it's easier for the crew to run in a locomotive than it is to run the length of the route backwards.
Back is the 60's a rail fan steam run broke a cylinder head and was like 5hr. late witch got all the other trains late also. I knew someone that was on it.
One of the main reasons is because modern coaching stock is electric heating, not steam heating, so the diesel acts as a generator for the heating system.
The Norwich to Yarmouth train has a diesel front and rear as there is no run-round at Yarmouth and the branch is not electrified like the main line from London Liverpool Street to Norwich. A friend recently saw two class 37s sandwiching three coaches.
