Well, it gets pretty complicated mathematically because there is some calculus involved, but generally the steady, unchanging component of the magnetism does not seem to have reliable biological effects (some people disagree, but they have failed to produce a convincing argument or data)
It is the changing part of the magnetic field that matters when it comes to eliciting biological responses. And what really matters is how fast it changes (dB/dt).
But the intensity of the magnetic field is the mathematical product of how fast it is changing, and how long it is changing:
I = dB/dt * t
This is the same as saying that the distance a car travels depends on its speed and how long it drives:
distance = rate multiplied by time (d = r * t)
Keeping in mind that magnetic fields drop off very quickly (~ 1/r^3)
To get the rate of change you need (dB/dt) for a biological effect, you need to quickly achieve a very high magnetic field intensity. As you get farther from the magnetic coils, this gets harder and harder to do.
Therefore, to produce a magnetic pulse with sufficient rate of change (dB/dt) to induce a biological effect, for deeper tissues (farther away from the coils) it is necessary to have very high peak magnetic intensity.
So, it is not that you need high magnetic intensity per se, rather you need to achieve sufficient magnetic field change rate for sufficient time, and the mathematical byproduct of that is the need for a high peak magnetic intensity to reach larger distances from the magnetic coils.
But the use of “carrier waves” to get magnetic fields to penetrate to deeper tissues makes the assumption that it is the peak magnetic field that matters, which is in my opinion incorrect. It is my scientific opinion that this technique is used (or it is just claimed to be used, after all, how would you really know for sure) as a way to attempt to give technical credibility to a PEMF system that has demonstrated that it actually does not work well for deep tissues.
I have done some experiments with this, and with fixed magnets (very powerful ones), and in no case did I see any enhancement of effects by adding either steady or carrier magnetic waves.
I have also never seen any reliable scientific report of the use of carrier waves to enhance the effectiveness of PEMF in any experimental system.
So, I could be wrong, but I think the use (or the claim) of carrier waves is just wishful thinking or deceptive marketing on the part of some PEMF manufacturers.