That is, if the preamp amplifies the instrument signal, why is a poweramp necessary at all?
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Amplifiers are not perfect. In an amplifier there are factors and constraints at play other than just amplification, such as distortion/fidelity, energy consumption, input and output impedances, frequency response (not that relevant in audio I guess)… So different amplifiers are optimized for different situations.
Fidelity and energy consumption for example are usually directly contradicting requirements (measures to improve one, make the other worse) (talking about amplifier classes here). Among other differences then, the preamp is a higher fidelity, less energy efficient amplifier, outputting a signal better suited for simple processing (filtering and other effects) (the low efficiency is not grave because the signal is low power), while the power amp is lower fidelity, but is more energy efficient, suitable for driving the speaker loudly with high power signals.
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