They were useful in constructing hyperspace-capable vessels, like the experimental Hith craft Skel'Ske, as icarons "preferred" to be in hyperspace; there was a much smaller energy demand placed on a ship when it entered hyperspace and took no energy to remain there.
Most societies banned the development of icarons because they caused paranoia and psychotic behaviour in carbon-based lifeforms. They were prohibited by the Fourteenth Resolution of the Armageddon Convention. Individuals who underwent a body bepple were particularly at risk. (PROSE: Original Sin)
Another variety, strange icarons, had no harmful effects on organic lifeforms. However, they did interfere with starship jump engines. If a ship entered or exited subspace in an area flooded with strange icarons, their engine would explode. When the Daleks invaded the solar system in 2157, they set up a strange icaron generator on Mercury to prevent aid from arriving from the Earth's colonies. (PROSE: GodEngine)