Can anyone explain the purpose of measuring light polarisation, as carried out by the upcoming planck satellite?
Polarized light carries information about the geometrical and optical properties of the source or medium
Many a times in observational astronomy, we need information on the interstellar/intergalactic medium and its scattering properties. That is observed by how it affects the polarization of light from a source behind it. That was one example. Here's a link:
The above link explains some reasons which might cause polarization of light. And these are employed to study the properties of source and medium.
On a personal note, though, there are no limitations. There might be other reasons which polarize light, yet to be undiscovered. It's a delicate property.