something to do with adherence.

TSVs are usually filled by Electrochemical Deposition or Electroplating of Cu, therefore a Cu seed layer is needed for the Electrochemical Deposition process to initiate.

Normally seed layer for IC vias is tungsten. Copper plated directly on silicon may diffuse to active elements and destroy them by creating deep-level defects. Of course, for TSV distance to active circuitry may be large, hence copper  seed may work as well

you do need both a barrier layer and a cu seed layer...