Echo, the nymph, is a blank. We do not know her. In Ovid, she acts (speaks) but is not noticed, as Ovid gives her no personality other than unconditional love and timidity, only to give herself to an unconditionally conceited Narcissus. We know Narcissus too well; indeed there is nothing more empty in us than when we echo Narcissus.
But echo, the sound, need not be a blank. It carries additional information: the time and strength of the delay, the resonances of the intervening spaces. And that additional information is key to its musical utility, imitation being the basis of some of the most elementary techniques for creating musical complexity, with timing, strength, re-iteration and additional forms of transformation available to create a field of echo-based forms, from call and response and canon to the most elaborately and densely networked and transfigured forms of imitation.
In my electronic music youth, every manner of echo (from spring reverbs and echoing acoustical chambers and tape loops to the first commercial analog and digital delay systems) was useful and exciting. But eventually — and, not coincidentally, as everything got, technologically, much easier — some boredom set in. Echo had become too familiar and the imagined opposite of an echo — a system with prescience rather than memory — was impossible.