If there is zero chance of getting a conviction all impeachment does is benefit the accused when they are found Not Guilty in the Senate trial.
Just ask William Jefferson Clinton who received a 73% positive job approval rating the week that the House impeached him.

It requires that the opposition power to the White House controls both branches of Congress, which occurs comparatively seldom, and requires a fairly strong degree of outrage. It has NEVER resulted in a president being removed from office. Nixon resigned not because of the threat of impeachment but because of the likelihood of conspiracy charges being laid against him. Clinton WAS impeached, but was acquitted in the Senate, and even in the unlikely event that Trump would have been impeached and been brought down in the Senate, it would have put Mike Pence in the White House.

The Mueller investigation has actually been very successful, in that it has unearthed a great deal of other activity that will likely end up with Trump and much of his family either destitute or in prison for a long time to come. It has also raised a lot of questions about the legitimacy of Trump's administration and given the Democrats in Congress considerable ammunition leading up to 2020. the report itself WILL leak out (I have no doubt about that), which will further weaken the legitimacy of the White House.

Had Pelosi gone with the option to impeach, it would have ultimately required a vote, which would likely have passed the House and failed in the Senate, and which would have effectively given Trump a new lease on life. Now, the House can continue to investigate to their heart's content (something that the GOP did during the Obama era on the slightest pretext, mind you), surfacing malfeasance during the lead up to a mid-term election going into a recession.

Personally, if I could change the Constitution, I think one of the key things that I would add is the ability to force a vote of no confidence, which would in turn call for new elections. It acts as a far more effective check than impeachment, and adds a degree of risk to the party out of power that they may not necessarily benefit from such a vote.

I think that's how it's written.