Enschedé used to have a lot of problems with its coil production. In the old days, you see, coil strips were made out of ordinary sheets, using the selvedge or margin of the sheets to stick the strips together to form a long coil. But the problem was that these strips used to tear too easily, because the sheet perforation was not up to the coil job. As these coils were used in stamp dispensing machines, this meant jamming machines and throwing your good money in without getting stamps etc., so something had to be done! And lo and behold; something was done!
In 1925, Enschedé came up with this luminous idea to remove a few pins from the perforator so that the stamps would not tear so easily. This is how the "interrupted perfs" came into being.
Later, in 1927/8, there appeared to be a demand for horizontal coils as well (rather than just vertical), so Enschedé improved on its idea and introduced interrupted perfs on all four sides, so horizontal or vertical coils could be made from the same sheets.
And it was not just the Wilhelmina definitives that were given this treatment, the "Carrier Pigeon" low value definitives, too, were made available for coil stamp dispensing machines. And even certain commemoratives, notedly the child welfare stamps, were produced thusly.
The horizontal coils were not a success, however, and were soon discontinued. A new type of vertical coil perf interruption was then introduced but soon after, the normal sheet perforations were changed in such a way that the strips were strong enough to be used as coils so the interrupted perfs were discontinued altogether, but they're still a nice field to collect with strips and blocks coming at a premium.