I frigging love grain.
I love it on my film photos: actually, I push them 1 or 2 stops (and even farther) not only to increase the contrast but also to get more grain. Some of my favorite images (like the one above) are full of big, rough grain.
This is why I add it to my digital photos as well.
Many will disagree, and that's fine. Photography has a million different purposes and not all ideas can be applied to every use case.
When it comes to fine art photography, personal photography, street photography and other similar genres, I believe the aesthetics of the image become much more important than the number of pixels and amount of detail shown.
Take this image as an example and compare the version with grain to the one without it. I show a crop of it as well, to observe the effect grain has even better.
The grain adds texture, some life, makes the image feel more... organic. More gritty and less digital. The fact that I added it using software is irrelevant.
Every image is different though. I think grain works better when it gives some texture to homogeneous portions of the image that have continuous tonality: think sky, water and other things like some buildings.
Grain also comes in handy when we want to "hide" detail that adds nothing to the composition. An example would be the image below.
There are plenty of other small adjustments to this image, but grain helps keeping away details from all the trees, branches and leaves present in the dark part of the photograph. I wanted the tree to stand out, and grain helped me to achieve that.
Someone made a comment when I released it: "this photo has too much grain for that sensor". I took the exposure with an A7II, a full frame camera, at ISO200.
Grain wasn't a technical issue with the camera, or a lack of skills. Rather, it was a creative decision.
Below, some of my images created on 35mm film that wouldn't be the same were they sharper and had no grain:
I believe we should embrace all the tools we have available (in both worlds, film and digital) to create what we want to create. And grain (artificially added or not) is one of them.