I don't know if there is a real advantage,,,
I always thought they were just cheaper to manufacture.
Now having said that I should say I own two modern single-shot .22 rifles,,,
One (Henry Accu-Bolt
) has a MSRP of $399.95,,,
But it is
a scary accurate rifle.
My other single-shot rifle is a H&R Sportster
It's a very nice rifle that retails for around $175.00,,,
It too is a scary accurate rifle.
Both of these rifles (when fired by a local sharpshooting lady),,,
Are 1-hole rifles at 50 yards distance.
I can't tell you that their accuracy is because of their single-shot design,,,
But both rifles have more inherent accuracy than I can utilize.
The real reason I bought them is because I simply have a liking for single-shot rifles,,,
It's a very leisurely way to spend a few hours at the range,,,
No frantic firing just to be pulling the trigger,,,
Just the quest for the one shot hit.