Losira from "That Which Survives" was a projection, yet seemed to have substance. Was she a holodeck-type projection with force beams to create "contact," or was she an actual object, transporter/replicator-type?
Then there was Landru from "Return of the Archons," a see-through specter hanging in the air.
Maiman built the first LASER in 1960. I'm not sure when the first hologram was recorded after that. While it is possible for sci-fi writers to imagine anything, some technical grounding in reality, such as LASER holograms, helps advanced technologies seem more "real." (In other words, LASER holograms were extremely new, at the very least, at the time of TOS.)