The Democratic point of view, per conversations with House Democratic aides working on the impeachment inquiry:

Their goal is to have Ambassador Bill Taylor, the top U.S. diplomat to Ukraine, and State Department official George Kent — both apolitical career officials — lay out what they see as Trump’s abuse of presidential powers to force a foreign government to interfere in U.S. elections on his behalf.

They will also argue that the witnesses make it clear that there was a quid pro quo, and that the actions carried out by Trump’s top officials were not normal.

The bottom line: Democrats do not anticipate the Taylor or Kent to provide any new information that they haven’t already described behind closed doors. However, they think it’s imperative that the American people hear directly from the key figures involved in the Ukraine saga.