The purpose of a fuse is fire prevention.

In your car the branch fuses (radio, fan, wipers, lights) are sized according to the current carrying capacity of each wire. This keeps the wire from burning if there is some sort of fault along the way. The source wire to the block of fuses might have its own fuse. The size of this fuse is determined by its own wire size (again for fire protection of that wire) and this fuse does not need to be the sum of the branch fuses -- unless there is a high probability that every branch will be drawing its maximum allowed current simultaneously. For example, a radio connected to a 20 Amp branch circuit might only draw a few amps while operating. Further, the dome lights are usually OFF while driving.

The primary wire connected to the battery terminal is usually not fused because it is sized to carry enough current to run the starter. As such, even if there is a large fault on that wire, the battery voltage will quickly collapse and limit the current.

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You can help yourself to understand what is happening in a battery circuit if you concoct a "practical battery" by drawing a small resistor in series with a perfect battery. There will be a voltage drop across that series resistor (I*R). The imaginary perfect battery will maintain its voltage, even if it needs to supply infinite current. The series resistor represents things that happen inside a real battery that will reduce the practical battery's voltage and limit its current output under various circumstances (temperature, internal connections between cells, overall condition, age, number of charge/discharge cycles, remaining charge, recent current draw history, etc.) When you attach your voltmeter to the battery's terminal, you are measuring the practical battery. Even with no apparent load, you don't get to measure the perfect battery because there is leakage inside the battery case and your voltmeter draws a small current.

[edit]: grammar and minor improvements