If nothing else, it increases the time to attack and own the system. Networked modules are more efficient and higher performing than old systems. This is the price of progress.
Just one example is the ECU. Old analog engines were crude and inflexible. Simple environmental changes would cause engines to run out of their efficient zones and dump more or less fuel than is appropriate for the conditions they’re experiencing. Modern engines take pressures and temperatures (from several locations) into account, along with throttle desired by the user and calculated load to change the engine parameters on the fly. This is why a modern Mustang can hit 30 mpg on the highway with 500hp and the 80’s model struggles with 20mpg and less power than a current Civic.
These ECUs can be the difference between safely driving and unsafe unintended acceleration into a truck in front of you. We haven’t seen any attacks which turn ordinary occupied vehicles into missiles… yet. I have absolutely no doubt that we will experience one in the next 10 years. Encryption and security may be the difference between this being a rare occurrence conducted by powerful nation state actors and something script kiddies can perform with a laptop and a weekend.