conventional forks are basically a piston in oil with a hole in it. This works fine so long as all the bumps you encounter are the same size and "sharpness" I'll get to that.
Basically there's two (four) kinds of damping you care about. High speed damping and low speed damping (compression and rebound for each)
The damping speed doesn't refer to how fast the bike is going, but how fast the forks (or shock) are being compressed or expanding. low speed damping are things like brake dive, rolling bumps in the road, going into a turn, etc.
High speed damping is things like railroad tracks, potholes, pavement cracks, pavement transitions. Stuff with sharp edges. Note that it doesn't matter how *big* the bump is, just how sharp the edge is.
When you hit a sharp bump, the wheel has very very little time to acomodate it, so the forks have to move very fast, but not for very long. when you hit a soft bump or the brakes, there's a bunch of time to take up the energy, so the forks don't have to move as fast (even though the bump or compression amount is larger)
Why does this matter you ask.
Conventional forks are basically a piston with a hole in it in a bath of oil. simple, easy and cheap to manufacture, and good enough for 13 years ago.
The problem with this setup is that oil going through a hole has very non-linear damping. when you move the forks slowly (low speed damping) the oil has almost no resistance, so the damping rate is very low. This means the bike dives under braking, wallows in corners, and rolling bumps in the road make the front end kind of pogo.
"use a thicker oil" I hear you say. Not so fast.
liquids essentially don't compress when under pressure, and the resistance to flow through a hole as the speed increases goes up exponentially, not linearly.
That means that if the damping force at speed x is 1, the damping force at speed 4x is like 16, and at 8x it's like 250. So, when you hit those little pavement transitions almost the entire shock gets transmitted right through the damper rod into the fork tube into the bike and into you, upsetting everything a bit.
That's with the same oil that was too soft before. You end up with a fork that's both too soft and too hard at the same time.
Cartridge emulators have a spring loaded valve to do compression damping, so the valve opens more as the speed becomes higher. you end up with a damping curve that's much more like a straight line than a curve toward the sky.
This lets you run a thicker oil for better rebound and low speed damping, and still have a plush ride, all of this works to keep the front end of the bike and the wheel stable.
I just put emulators in my friend's GSF400 using the stock springs and after the test ride he thought there was something wrong with the shock, as it felt really harsh compared to the front end.
Yea, get them. They're worth it.
'course, if you're going to spend $240, get an F3 front end and just swap parts, stock look, F3 suspension, and real cartridges, and it's less work because you don't need to drill out anything.