This might be coopted from woodworking, a practice much older than photography. Google returns the following:
The quote "It's called grain. It's meant to be there" appears in the context of woodworking, specifically referring to the natural grain pattern in wood. It emphasizes the inherent structure and beauty of the wood itself, rather than trying to alter or conceal it. The phrase suggests accepting and appreciating the natural characteristics of a material, rather than striving for perfection or uniformity.