So it's only a random LFO when you feed it noise. Also, if you think your synth has one sample and hold, you might be wrong. In analog monosynths the keyboard CV is controlled by a sample and hold circuit fed by the keyboard. Remember that the keyboard itself isn't doing anything after a key is released but notes must continue (with duration controlled by Release). It's also used in conjunction with the portamento control. Polysynths like the Jupiter-6 have tons of sample and hold circuits for various CVs for each voice.
To see a simple sample and hold circuit and description, see "SAMPLING" here. The Rogue uses a variation of this.
The "In" is coming from the keyboard. It is the current key's voltage or open when no keys are played. C3 is the capacitor that gets charged and holds the keyboard voltage. It is also used with R13 to form a low pass filter for portamento a.k.a. glide. R12, R14, and R15 are there for protection since the CV in and out are connected to a jack on the back. Someone could inadvertently fry the op-amp by plugging in something they shouldn't.
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