They are quite useful in fairly loose soil, but not necessarily so, in stiff / hard clay.
Others may have graduation marks cut into the shaft to assist in determining planting depths.
History has it that dibbers in the form of a pointed stick have been in use since Roman times, and have remained mostly unchanged since.
Probably the main differences today is that they are often formed of hardwood and that have been turned smooth on a lathe and had a Tee handle fitted.
Alternatively they are made of moulded plastic.
A recycled broken spade / fork shaft that has been hewn to a point makes for a cheap alternative.
Pro's and Cons:
When planting seed potatoes, dibbers can have their drawbacks, e.g.
The pointed end makes the hole terminates in a point, the result being that the potato tuber when dropped into the hole, does not fall to the bottom, meaning; the roots it sends out will have to grope for a time to find anchorage.
Similarly, this drawback can affect transplanted plants that have a fairly large root ball as again the roots may not reach the bottom of the hole.
In conclusion this tool has its uses, but a hand trowel is often the better option.