Robert Mueller has already said that his coerced testimony before Congress will not add to what we already know from his report.
So why are the Democrats doing this? The leadership in the House has said that they have no plans for impeachment. Most Americans have essentially forgotten a report that most people have not read, including members of Congress.
This is a stunt by Democrats intended to get some kind of advantage over Republicans for the upcoming election in 2020. Pathetic. They have not even taken the Trump administration to court for all the subpoenas they've ignored.
Democrats want to look like they are going after Trump when they are doing nothing. It's all about the next election. And currently, Trump is on pace to win reelection.