The polarization of CMB anisotropies has yet to be detected (as of May 2001) and as we shall see is expected to be very small. So why bother measuring it?
The polarization, unlike the temperature anisotropies is only generated by scattering. When we observe the polarization we are looking directly at the so-called last scattering surface of the photons. It is therefore our most direct probe of the Universe at the epoch of recombination as well as the later reionization of the Universe by the first stars. The latter can really only be probed by the CMB through its polarization.
Polarization serves as a fundamental consistency check on our interpretation of acoustic peaks. The shape and amplitude of the acoustic peaks in the polarization can be precisely predicted from the temperature peaks under the standard inflationary paradigm. Any other explanation of the peaks that do not involve acoustic oscillations would violate this relation. Furthermore, since the polarization isolates the recombination epoch, one can directly search for correlations larger than the causal horizon back then and form a sharp test of our inflationary paradigm.
The polarization, which carries directional information on the sky (as a tensor field), contains more information than the temperature field. Measurements of the polarization power spectrum can greatly enhance the precision with which one can extract the physical parameters associated with acoustic oscillations.
Furthermore, the polarization through its directional information provides a means of isolating the gravitational waves predicted by models of inflation. As such polarization provides our most direct window onto the very early universe and the origin of all structure in the Universe.