Thats a good question, and your right there should be a good reason to justify a plugging in which is aimed at a feature set C4D has. I will address this in more detail in the video. What I can say on a simplified level for now is Ornatrix takes off where C4D hair system end in its abilities.
This is mainly around control over particular guides, hair groups, its consistent interpolation methods, and its powerful modular approach.
Modular:
In C4D you have a single material with one set of attributes that change the outcome of the hair, these are called Operators within Ornatrix, lets take Frizz as an example. You can make and apply a map to control how much is applied and to where in C4D native hair system. This is also true with Ornatrix but things can taken much further. Due to Ornatrix modular methods you can apply as many operators as you wish in any order.
A frizz operator is applied to the operator stack. like C4D it applies to the the entire hair. You may want one type of frizz on the top of the head, and another type on the sides. The only way to do this in C4D is to make multiple hair objects to have multiple materials. In ornatrix you can simply select the top of the hair guides, make it into a group, invert the selection make the group for the sides. Add two frizz operators and apply one to one group, and one to the other. No texture maps needed.
What if within the settings of the frizz you wanted some hairs to receive more frizz not just the full length but just at the tips of the length? This can not be done in C4D, its all or nothing. You can use texture maps for the entire length in C4D but not on a per vertex level. With Ornatrix not only can you assign a group such as the top of the hair only, but within that group you can assign particular guides to receive more or less frizz, again no texture maps needed. These are called channels and can be painted with a vertex brush directly on the guides, or flood filled either as a value between 0 = Back to 1 White and all values between.
You can make as many Channels as you wish either on a per strand basis or a per vertex basis. If you use a per strand basis the assigned value applies to the whole guide. A value of 1 would get 100% frizz, 0.5 = 50% all the way down to 0 no frizz at all. If you make a channel on a vertex basis you can brush with the vertex brush tool any value you wish down the length of the guide. This lets you apply frizz right at the tips for example.
Not only do you get this additional control but no texture maps are needed. You can have has many frizz operators as you wish, this applies to all operators in which some operators gives you control on not just the amount but other attributes.
Clumping:
Clumping is essential to get realistic looking hair or fur, C4D clumping abilities are not only limited but very hard to use. In reality there are multiple layers of clumping from larger clumps to very fine layers. C4D only allows you to add one layer. With Ornatrix you can add multiple clump operators each one can take the last into account to further split larger clumps into smaller ones. You can again choose a group of guides, and particular channels for individual guides. Where Ornatrix takes things to a whole level higher is you can choose individual hairs and clump them, or remove clumping, or edit individual clump sets. You have so much control over clumping that C4D does not.
Direct Hair grooming:
You can make your groom using guides, add multiple operators to fine tune it, then actually groom the hair directly. This means you can use all the brushes directly on the hair itself.
Bake operators to your guides:
You can force certain operators to be baked to the actual guides, this means you can remove them operators which makes simulations for example faster. Remember you can apply operators to both the hairs or directly to the guides.
Interpolation:
The interpolation for how the the hair responds to the guides is essential for a good outcome. C4D is lacking in this area. Ornatrix really shines here. Some styles need different interpolations and Ornatrix gets you so close to the style your making with the guides.
Other mentions:
The Surface Come tool lets you draw arrows in the direction you wish the hair to flow. v2 has made this even better but C4D does not have this feature. This is especially useful for animals, but can be used on human hair. Push Away From Surface will push your hair away from the head, this is a one click fix for intersections. It works and works well. You have further operators to help add additional details such as more verts to certain strands, smoothing, collision prevention. You can save hair styles for later use, export for Unreal.
These are just a few things to mention. The outcome is that you get the hair under control spot on to how you want it.
Dan