It was quite un-natural of Democrats when they had the upper hand of having a witness in Trump's second impeachment trial, to not use the witness.
They initially voted to include the witness in the trial and later reached the deal.
What deal was struck?
The deal is just that: Rep. Jaime Herrera Beutler's statement would be entered into the record, no witness will be called, and Republicans will refrain from obstructing Senate business.
After a two-hour break in the trial following a Senate vote to allow for witnesses, House managers and Trump's attorneys agreed to stipulate that a statement released Friday by Rep. Jaime Herrera Beutler, R-Wash., could be entered into the trial record. The deal averted a showdown between the two sides over whether to call Herrera Beutler and possibly many other witnesses — a development that could have delayed the trial's conclusion and the Senate's other business for weeks.
Your real question is what Democrats got in return for not calling witnessed. To understand that, you need to understand a bit of context about how Impeachment trials are run. According to the Senate's impeachment rules, the Senate must suspend all business while the trial is underway. Doing anything else requires unanimous consent from all Senators:
Senate committees may hold hearings in the morning of each trial day, but doing any business such as sending bills, nominations, or other matters to the full Senate would require the consent of all senators.
The Senate impeachment rules provide that the chamber must suspend its legislative and executive business while the trial is under way.
This posed an obvious challenge for the Biden administration: they need to get their nominees confirmed, to say nothing of the looming March 14th deadline to pass a COVID-19 relief bill and extend unemployment benefits. If the Senate was barred from doing business for weeks or months right at the beginning of his term, the Biden administration would be hamstrung and the country would lack leadership for a long period.
To prevent this, the Democrats made an earlier deal with Senator McConnell to allow the Senate to split time between impeachment and normal business:
Democrats, poised to take unified power in Washington next week for the first time in a decade, worked with Republican leaders to try to find a proposal to allow the Senate to split time between the impeachment trial of Mr. Trump and consideration of President-elect Joseph R. Biden Jr.’s cabinet nominees and his $1.9 trillion economic recovery plan to address the coronavirus. ...
Democrats faced the vexing task of trying to manage a trial just as Mr. Biden will take office, and as they claim control of the chamber. Once the House formally sends its article to the Senate, a trial must commence almost immediately and rules dictate that all other business come to a near immediate halt and remain frozen until a verdict is reached.
Senator Chuck Schumer of New York, the Democratic leader, worked on Thursday to agree with Mr. McConnell on trial rules that could get around those strictures. The goal was to divide the Senate’s days so the chamber could work on confirming members of Mr. Biden’s cabinet and considering his stimulus package in the morning and then take up the impeachment trial in the afternoon.
With this deal, the Senate was able to balance the impeachment trial with their work on COVID-19 relief and confirming Biden's nominees. The surprise vote to allow witnesses, however, threatened this deal. A number of Republicans threatened to withhold their consent for the "dual-track" Senate business and to drag out the trial to block the consideration of Biden's nominees if witnessed were allowed:
[Joni] Ernst even warned Democrats , “if they want to drag this out, we’ll drag it out. They won’t get their noms, they won’t get anything.” Ernst was referring to the majority of President Biden's cabinet that is still waiting for confirmation.
The "deal" on witnesses was a compromise to satisfy these Senators: in exchange for not calling witnesses, Rep. Jaime Herrera Beutler's statement would be entered into the record and the earlier deal on a "dual-track" impeachment would be preserved, allowing the Senate to continue the important business of confirming nominees and passing COVID-19 relief.
