Input is that which is, well, put in—whether literally, as in "the input from the guitar to the speaker," or more conceptually, as in "the group leader wanted everybody's input."
The verb input, in the computer sense, didn't exist much before the late 1940's when it became common among computer professionals, who input data into their machines. Why they weren't satisfied just to "put in" the data, we may never know. Before that, the noun input was an economic term meaning anything that went into production: the grain was just one input among several required for the production of cereal. But often, these days, input means a contribution of some kind, usually a thought. You'll thank me for my input later.