Spline sockets and wrenches were originally designed for actual “Spline Head” fasteners. The spline-head fasteners usually show up in aerospace or military use. Both Proto and Snap-On manufacture Spline wrenches and sockets, although the prices aren’t cheap.
I don’t know whether it was part of the original design, or whether somebody just stumbled upon the feature, but you can also use the spline wrenches ans sockets to turn 12point, 6point, torx, and 4point fasteners, and maybe some other weird festeners as well. Due to the rounded spline design, the wrenches grip further away from fastener head corners so the wrenches are less likely to round fasteners.
I believe old-SK used to manufacture spline sockets in both metric and Fractional sizes. Most other manufacturers seem to manufacture just “metric” sizes, that are dual-sized for both metric and fractional “equivalent” sizes. I’m not sure whether there are officially both metric and fraction spline bolt head sizes. Since the wrench and socket sizes don’t always specifically match the head size, there might be slop between the wrench and festener head, although the spline tools can still be used for tightening and loosening a festener, it’s just annoying if you have to finely adjust a fastener.
If you buy a set just make sure it has as many wrenches as possible to avoid gaps.
For innexpensive wrenches they’re not bad, although I would still want to have a regular set of wrenches sockets to go along with the spline.