The more compelling reason to own a tube buffer, is to help give component hooked up to "see" an impedance you want the component to see. I am not an EE or anything close, I hope some one with a clearer explanation to chime in. In my system I have used certain buffers to aid impedance mismatches. Almarg please...
Yes, that will often be the case. This thread
provides a good discussion of impedance matching, in the context of line-level analog interfaces. As I said therein:
Ideally the input impedance of the amp should be 10 or more times greater than the output impedance of the preamp, at the frequency for which preamp output impedance is highest. Otherwise audible frequency response irregularities MIGHT result.
If, as is often the case, the highest output impedance of the preamp across the audible frequency range is not known, and only a nominal output impedance is specified (perhaps based on a frequency of 1 kHz), I suggest using a ratio of 50 or more, and preferably 75. Many tube preamps, and some solid state preamps, use a coupling capacitor at their outputs, which can cause their output impedance to be much higher at deep bass frequencies than at higher frequencies.
Impedance incompatibilities are most likely to be encountered when using a tube preamp with a solid state power amp.
If the power amp has an input impedance of around 47K or more, it is unlikely that there will be an issue, even with a tube preamp.
A tube-based stage will usually have a high input impedance, which is in the direction that may be helpful. If it is intended to be used as a buffer, it will probably also have a low output impedance, which would also be in the direction of being helpful. But the opposite might be the case, with respect to its output impedance, if, for example, it is a tube preamp that is not specifically intended to be used as a buffer.
Based on my experience, I agree with all of your (Mechans) comments regarding tube power amps and preamps. Although of course opinions on that will differ widely. And of course many speakers will not be good matches for tube power amps.