Ferrites meant to prevent or reduce RFI do a better job if they absorb RF energy.
Also, if it isn't lossy, the inductor formed by this will resonate at some frequencies, not a good thing if you are trying to reduce RFI. The lossy ferrite core decreases resonant peaks
I should say that NOT ALL ferrite beads and toroids for reducing RFI are lossy, but many are.
Noise In, Heat Out
Recall that ideal inductors and capacitors do not dissipate any energy; they merely store energy, either in a magnetic field (inductors) or an electric field (capacitors). A resistor, on the other hand, takes energy out of the circuit and dissipates it as heat. Ferrite beads, unlike inductors, are intentionally resistive at high frequencies. This is why the above plot has the red dotted line labeled “R”—from about 100 MHz to 1 GHz, the bead exhibits significant resistive impedance, not reactive impedance. Actually, some ferrite beads and ferrite-core inductors are almost identical in construction, except that the ferrite bead uses a more “lossy” ferrite material because the manufacturer wants the bead to dissipate rather than store high-frequency energy.But why belabor this point? We belabor for two reasons. First, you cannot truly understand a ferrite bead until you have adequately pondered this fundamental distinction between an inductor and a bead. Second, this “lossy” characteristic makes the ferrite bead especially suitable for noise suppression. Why? Inductance can lead to resonance and ringing when high-frequency noise energy stored in the inductor interacts with capacitance elsewhere in the circuit. As we saw in the previous articles, ringing can become seriously problematic even when we are dealing only with parasitic inductance. We don’t want to exacerbate the resonance/ringing situation, and thus we opt for ferrite beads over inductors.
