For many years, planers and jointers had straight knives. There was no option. You had your knives sharpened when they got dull or bought a grinding attachment for your machine and sharpened them yourself.
I remember when the first spiral cutterhead appeared, the Neuman helical head. It had four continuous knives that wrapped around the cutterhead. They were expensive! A 25 Delta planer with conventional straight cutters cost around four grand. With the Neuamn head, the price jumped to almost nine grand and that was not including the extra $1,500 for the grinding attachment. And you really had to get the grinding attachment because there were very few sharpening shops equipped to handle a spiral knife. Plus, the things were a PITA to adjust.
For many years now, we have had spiral insert cutters. These heads use "chips" instead of continuous knives. The chips can be rotated four times to provide a fresh cutting edge. There are a lot of advantages to these cutterheads. For one thing, if you nick a cutter, you only have to rotate a couple of chips instead of the whole set of knives. In a production setting, there is less down time because it's faster to replace the chips than to install fresh knives which then have to be tediously adjusted.
The helical insert heads are not cheap. They are available for most planers but sometimes can cost as much, if not more, than the machine itself. The insert chips are not exactly cheap either. If you figure a hundred and twenty or so chips per head at three to five bucks each, even considering four edges per chip, that's a lot of sharpenings of conventional straight knives before you "break even." Also, these heads are prone to "ridging" because the cutter is not continuous like the Neuman heads were so the surface quality is not as good. Nevertheless, they still offer an advantage over straight knives when cutting difficult woods or highly figured stock.