Former diesel tech, general tinkerer, and someone who drives a diesel daily: it depends.
Good quality fuel from a station in newer equipment needs nothing generally. It isn't a bad idea to run an additive in newer common rail equipment to add lubricant to the fuel due to the extreme pressure and multi-pulse injection but its not required. In those applications the main enemy is contamination. A good cetane booster can be beneficial as well but again isn't necessary. I'm currently upgradinf the fuel system on my truck due to an early injector failure most likely due to bad quality fuel but couldn't exactly empty on the side of the road.
Biodiesel is different. If allowed to sit it can self contaminate by growing microbes or algae. It is good practice that if you store it or if you run it in a vehicle that doesn't replenish fuel often to run additives that keep the growth out. This is especially true if the fuel system is not air tight, or is prone to organic matter getting in like grass clippings.
Older diesels benefit the most from additives, because they were designed with a virtually different fuel in mind. They need added lubrication for the injection pumps. They also benefit much more from cetane boosters. But mostly its the added lubricant, shortening the life span of the injection pump.
I'm not a kubota tech and obviously do not know their entire engine line up so choosing to run additive may vary. However on my LX, and with every kubota diesel I've come across thus far which are all low hp at work, they are not high pressure and are designed to run on modern fuel so I don't see a benefit other than mabey an antigel in winter. I do run additive in mine, but only because my fuel tank is shared. The first two tanks were untreated and I experienced zero benefit to cetane booster.
Someone mentioned a marine application, I know for marine fuels similar to aircraft they don't necessarily meet thr same standards if you are fueling on the water. But I can't say for sure if they are using ultra low sulfer diesel or not. Does it say at the pump?