The scrap sharing is far more useful if you're building settlements manually. But it may also be helpful for tinkering at the crafting stations for weapons, armor, robots, etc., especially early in the game when I tend to be low on adhesive and screws and the like.
As for scrap donation - supposedly the cities will eventually accumulate enough scrap on their own to upgrade, even with Citizen Needs Requirement enabled, but I'm not that patient. I donate to upgrade faster. By the time I'm wanting cities to upgrade, I'm usually accumulating so much scrap that I need to dump it somewhere.
I don't use the vanilla provisioners. Instead, I use the logistics station. For purists, it's a bit of a cheat, but it's a quality of life improvement for me. It doesn't require local leader, so I don't have to always put 6 points into charisma. It also has options to collect chems, food, water, scrap, loose mods, and maybe things I'm forgetting from all settlements that contain a logistics station along with a single inventory that you can access from any built station without having to run or fast travel back to "home base."