Someone correct me if I am wrong, but typically Zobel networks are used in speakers to tame either uneven impedance curves or a mismatch in impedance. I believe, as Gluca said, that big spikes in impedance will cause uneven frequency response at the output of the amp and the zobel network evens out this effect.
If you choose the values correctly, a series CR circuit in parallel with a series LR circuit (O/P transformer DC resistance in series with leakege inductance) produces a purely resistive load at all frequencies. Since valve amplifiers use output devices with significant output resistance (ra), if the load resistance is not constant, it makes the frequency response deviate from flatness with frequency. This is usually tested by using a square wave to look for ringing (by far the quickest) or sine wave frequency response. Thus, as other posters have said, a Zobel network is there to prevent square wave ringing or peaks in the sine wave frequency response. The values have to be found by experiment.
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