Opal's glossy surface does more than add sheen. It gives the colour somewhere to move, reflecting windows, lighting and nearby surfaces back into the room.
This is particularly noticeable across a larger wall. Instead of seeing exactly the same colour from one end to the other, small changes in reflection and tonal variation give the surface a little more depth. In a bathroom, for example, this can work beautifully alongside natural light, mirrors and polished fittings.
It's also why a gloss tile doesn't necessarily make a room feel cold or overly bright. The surrounding materials have a big influence on what is reflected back from the wall. Warm timber, natural stone and softer textiles can introduce enough contrast to keep the overall scheme feeling balanced.