A circuit breaker is not just a switching device - its primary function is protection, its secondary function is switching / isolation.
The fuse may be on the source side of the circuit breaker to protect the circuit breaker from the pole mount transformer PSCC (prospective short circuit current) if it is higher than the circuit breaker PSCC rating. The fuse will act as the overall system protector, should excessive short circuit current flow due to system very low impedance to the fault location. A pole mounted transformer is usually an intake from the local power distribution company, at say 11KV, direct from one of their secondary substations. if it is only a short OHL feed to your transformer, the PSCC may be very high indeed and a fuse is the best fast-acting, current interrupting medium in these circumstances (a circuit breaker will take longer to clear any PSCC fault due to the mechanics of its operation, as opposed to a wire melt and that extended time could allow equipment / insulation on the downstream side to be over-stressed and damaged by the high current flow).
The 350A fuse rating vs the 400A circuit breaker rating will have been calculated to ensure the best co-ordination / discrimination between the two, whilst ensuring the complete system (including the circuit breaker itself) is adequately protected.
System coordination and discrimination is done from the load upwords to the supply and upstream not vice versa. For example we place a 16 amp fuse in the plug which is connected to a miniature of 20 amps this is only example. So the discrimination is from the load and up to the supply but in this case is vice versa where the supply is the transformer and the discrimination is in the wrong direction in this case.
The fuse / circuit breaker combination may be well specified to give the correct system co-ordination, discrimination and back up protection functions. I am of course making some assumptions here, as we do not have full detail of specified particular situation / system but I hope this helps with some understanding of what is going on.