Actual paper is both a sensory and a visual experience while a digital imitation is purely that. Paper can be as much part of the design process as the choice of type and color.
It comes in both uncoated and coated including matte, gloss, cast coated, and now the very popular soft touch that gives the paper a velvety texture. The uncoated sheets can also come in everything from smooth to linen.
When it comes to book publishing, the decision is usually made on what the surfaces will be holding from purely type to high end photography to original artwork. Bottom line is you get both the visual experience of the paper which can be subtle, to a sense of touch. All have an impact on the design, structure, and impact.
Dare I say that an agency that designs a digital version with these background textures can only be trying to add to the visual experience and taking a leap that it will somehow provide the sensory. But it really doesn't.
If you've never experienced the range of papers, what they feel and look like, you can always reach out to the paper manufacturers and they will be glad to send you sample swatch books for you to see and feel. As for choosing a paper stock for the ink on paper version, that's up to design and can often be constricted by budget. But very much worth a look and feel. Providing an actual sample to someone who you're trying to sell a print project to goes a long way to finalizing a design.
I don't know if I've answered your question. But as an old school print production and account manager who was born into the business when hot metal was still being used, I can say that there is no substitute for real paper.