It’s possible to get a higher proportion of good fruit, because the tree is trained so tightly that it’s kept open and none of the fruits are shaded by upper branches etc.
The trees are fully supported so they’re less vulnerable to wind damage.
The small size means that they can easily be covered by bird netting/fleece etc. (Obviously this applies to small bush trees too).
If grown against a wall then the stored warmth of the sun can help to extend the season and ripen fruit in colder climates.
Varieties not to grow like this are definite tip bearers such as Golden Russet.
All good points in the responses above…. I decided to go this route because I was moving from a spot where I had a few acres and enough space for full trees of as many varieties as I wanted onto a 1/2 acre city lot. I wanted to fit as many varieties in the small space as I could and wanted to have each variety as its own entity rather than multi grafting…. And I liked the esthetic of this style.
As far as varieties that work well: I think you’d want to avoid the ones that lean heavily toward tip bearing….. the “spurrier” the better I’d think, if that’s a word, lol