A properly built shower is waterproof before you install the tile. Industry standards call for caulk at the changes of plane. Mostly, this is to prevent high volumes of liquid water from getting into the setting surface. They are better served by water vapor and small amounts that may get there by wicking unless you use a surface waterproof membrane underneath the tile. A good caulk doesn't support mold. Now, that doesn't mean that soap scum, and whatever may not get ON it and then support mold, but the caulk itself (especially 100% silicon) doesn't support mold. Since it's hard to clean those crevasses, caulking them means less is likely to get in there to accumulate and then support mold.