Maybe the highpass filter is used when the pickup is a wideband microphone.
There is a park near my home where a beggar plays an acoustic guitar that has a microphone. It is amplified with a battery-powered amplifier which plays through a speaker. A highpass filter is probably used to cut traffic noise.

Is he any good?
If it's battery powered, it pays to filter out any frequencies that the speaker cannot reproduce, otherwise it is just battery power converted to heat.

 

Also, a "Super-Cardioid" Microphone will help a lot.
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The beggar played his amplified guitar through a good speaker in a proper enclosure, not through a tiny squeaker.
He played well and his speaker sounded good.