Gain stages are like the family jewels. They need to be protected.
To be specific, there are two types of gain stages: current and voltage. It is the latter type that is vulnerable to outside influences.
In fact, it is the current gain stage that we use to isolate and buffer the sensitive voltage stage.
A voltage gain stage has the most critical job to perform in any amplifier or preamplifier. It takes at its input a small amount of voltage—say voltage generated from a phono cartridge, microphone, or even a source component like a CD player—and outputs a much larger voltage clone of what is fed into it: 1 volt in = 10V out (for example).
The accuracy of the cloning defines the performance level of the voltage amplifier. Changes in input voltage frequency, level, phase, must be perfectly mirrored in larger amounts at the output. Anything less is a loss, anything more is distortion.
Even if one could design the perfect voltage amplifying stage the next problem is often more problematic. How to keep it isolated from outside influences. Anything external to the voltage gain stage becomes part of its circuit and thus detracts from its performance—robbing it of its family jewels so carefully crafted.
Which is why we place input and output buffers or circuitry to isolate and protect the family jewels.
