I'm going to get long...so you've been warned.
First what is valve clearance? It is a engineered clearance between the tip of the valve and the rocker arm (in this discussion...there are other designs as well). That clearance is there for a reason. One, it allows for wear, among other things, and something else--that clearance gives you an idea of whether or not anything is wrong, when you check it.
Valve clearance will change with usage and wear. That is a given. It will also change with thermal expansion. The valves run hot and they will expand. The cast iron block has a different thermal expansion rate than the steel pushrods, valves, rockers, etc. it's all different. We are talking micro and milli inches when we are talking about expansion, but the stack-up is what we're looking at (the combined growth of all of the parts). Say you have .007" clearance at room temp, say 65 degrees. At 200 degrees that clearance usually closes up a couple thousandths. Many think it opens up, but IME, it actually decreases. I think the major reason is because of the valves; they run hotter than the rest of the engine, especially exhaust valves. Now you have .005" hot running clearance. Kubota would like to have a cushion; so that if you run it hotter than normal, or if you're working it hard, that clearance won't end up at zero. You don't want zero. Not on these engines (they are all mechanical tappets-NOT hydraulic; with the exception I think of the M7 or M8? tractor). If it goes to zero, the valve(s) get hung open slightly and allow gases to escape through them, which wears the face of the valve, and the valve seat. Also heat is more likely to build up since the valve cannot transfer heat from itself to the cylinder head; and on a diesel this is pretty important since there is no liquid fuel going across the valves, which helps to cool them. Air only.
You also don't want the clearance to be loose either. Loose valves act like a hammer on the tip of the valve, the rocker arms, the pushrods, tappets, and camshaft. Millions of cycles of hammering on them, something will usually break. Often the valve, or rocker arm. Sometimes the tappets will break and that is never pretty. But then again neither is a broken valve, usually destroys the engine and sometimes spectacularly.
Kubota (and every other manufacturer that uses "solid" tappets) asks you to check the clearance. The biggest reason here is to detect issues before they swarm. For instance if you are checking a V1505 and Cyl#1 Inlet valve clearance is .030 and all the rest were .009, you just found a problem. However, A couple thousandths either way of spec is normal wear. Secondly to account for wear. If they start getting loose, they hammer the seats out of the head among other things. If tight, high pressure gases will escape around the seat/face and wears them, and usually results in a cracked head, dropped/broken seat, worn seat, severely worn valve face, burned valve face, cracked valve, broken valve, etc. None of which are good. And yes I've seen it on Kubota. cracked valves especially-and cracked heads. Lots of cracked heads but usually due to overheating.
Most folks don't maintain them, they never check valves. I think 800hr is the interval on a lot of them. Normally they're still pretty close. Run them twice that without checking, and they're usually starting to get loose or tight depending on how much wear there is. "Well it runs fine and ain't noisy so I ain't worried about it". Right. Can't hear the noisy looseness over the normal clattering of the diesel engine. By time you hear it, it's too late. Or when they tighten up, you never hear that either. Just starts getting harder and harder to start, keep spraying the starting fluid to it, next thing ya know its running on 1 less cylinder until warmed up (clearance opens up a little), then you think it's fine. Then one day you're cutting grass with the shredder on back at full load and rpm, wham bang clank, engine stops, and death fluids begin leaking out of the engine...broke a valve off, destroyed the entire engine (nothing salvageable in most cases, maybe the intake manifold and maybe the valve cover), all because ya didn't have time or was too lazy (that's usually my excuse) to check the valves. It really does need to be done. if you don't have time at 800.0 hours, write it down on your to-do list. If it goes over big deal. But it still needs to be done asap.
on a lot of modern gas engines with overhead and dual overhead cams, there are no rocker arms. The valves are actuated by the camshaft alone. The clearance is the gap between the cam and the "bucket". There is a metal cup that fits down kinda tight into a cylinder in the head, over the outside the valve spring(s), and the bucket usually has either a little shim between the tip of the valve and the bucket, or the bucket itself is made in such a way that it can be had in different thicknesses, to adjust the clearance between the bucket and the cam. IIRC the Sidekick 850 Kubota has this design. That is the only reason I mentioned it. They're a little more involved to adjust the clearance than a rocker arm style valvetrain but they also don't need adjustment nearly as often, as there are less moving parts and also a lot more stable valvetrain, which means higher RPM potential which means more HP.