Some rock guitarists, looking to add an acoustic guitar sound to their live repertoire, try plugging their acoustic into their electric amp, only to find that the sound is wrong. An electric guitar amp differs substantially from an acoustic amp.
Think of acoustic amps as small PA systems. They are designed to amplify a clean signal, and not change the tone of the instrument. Acoustic amps are solid state circuitry, to help minimize distortion.
Many keyboard amps function well as acoustic amps, and vice versa, because keyboardists want a clean tone coming from their instruments.
Electric Guitar Amps
Electric guitar amps are designed to color the sound of the guitar. The tone from a Fender amplifier sounds substantially different than does a Marshall or a Mesa Boogie. In this respect, they are part of the signal chain, so electric guitarists choose their amplifiers based on the sound they want.
Electric Amp Technology
Although solid state circuitry has advanced considerably, most electric guitarists choose tube amplifiers to get a smooth, "creamy" distortion from their amps. Solid state electric guitar amps are often much cheaper than tube electric guitar amps.
Because acoustic players strive for transparency above all, many eschew amps altogether, and either use a microphone on the instrument to get a true natural sound, or plug the instrument into a PA via a direct-in (DI) box. Many modern acoustic instruments have a blended option that blends a small microphone for natural sound with a pickup for sound reinforcement. These systems require a special preamp.