Well, Don [resqcapt13] and Nick can explain this better, or more accurate than me, but here's my $.02:
The fuses at the disconnect make up failsafe [somewhat
] running protection for the complete assembly.
This would ward off the evil overload spirits [mythological barbecue gods] from reaking too much havoc if the power system experienced a single phasing problem, or if the monkey wrench committed suicide within the compressor's belts / other exposed rotating items, while the unit is being serviced or troubleshot.
I'm just so use to installing fusable EXOs [disconnects], that it seems like "it's always done that way"...
The breaker is branch circuit protection [in case the forklift gods are angry and drive into the conduit for the AC] offering overcurrent protection [fault current protection].
It needs an HACR rating to be used on a circuit with the characteristics of an AC unit - high and extended Locked Rotor currents, LCL [continuous load which may increase in amperage, but seldom decrease under load], short off cycle durations, increased loads [multistage AC units], and all that baloney.
The fuses would be running protection for the entire machine, plus nearly always will open from a heavy fault or overload [EMT fuses are the exception
Nick has a better explanation than me, so go with that!