Conceivably, some mods that add content may need/want to make graphics or sprites for the new content (like say, new creatures), but these sprites can't fit in with every tileset; I'd guess that most will just style them for fitting in with the vanilla Premium tileset.
But there will also likely be some alternative tilesets that are very popular as well; the likes of Vordaks, or GemSet or what-have-you. It would be nice if a mod could "support" multiple tilesets, so that someone can use the mod with both Premium and "popular graphics pack X", and have a cohesive/good looking experience either way, and the mod author won't have to post a bunch of supplementary "mini graphics packs" into the workshop or DFFD.
The most basic version of such support would be letting the player see a configuration menu for the mods where they tick "X graphics" for the mod. But this would be rather cumbersome because:
1. It would need to be done manually for every mod.
2. It would need to be done every time one switches graphics packs.
3. Since a player might use different graphics for different saves, they would need to configure it per save.
So... That's not a great solution.
So the ideal way of allowing mods to support different graphics packs, would be to let the mod author designate a folder for a given graphics pack via whatever mod ID that graphics pack has, and then if the player is using vanilla Premium, the game will load whatever graphics folder the mod author has designated for vanilla Premium, and if they are using GemSet the game will load whatever graphics folder the mod author designated for GemSet.
If the player is using a graphics pack the mod doesn't have a designation for, there would be one set of graphics defined as the "fallback" graphics by the author.
It might be a good idea to let the mod author designate multiple mod IDs to use the same set of graphics, like making CLA use the same graphics the modder created for GemSet for example, in case they don't want to go to the effort of properly supporting CLA (ie they don't want to draw graphics for it), but think it would be more fitting to use GemSet's graphics in this case instead of vanilla Premium.