The non-release of video evidence prior to the trial's completion would be to prevent potential jurors from being tainted, because jurors are required to obtain their knowledge of the facts of the case solely from what is presented to them at trial. Someone who saw the full video prior to the trial would be ineligible to serve as a juror in the case in much the same way that witnesses of a crime can not serve on the jury in a trial arising from that crime.
The fact that he was acquitted meant that the case could never be retried. If he was not acquitted, the video probably would have stayed under seal until all possible appeals were exhausted to prevent jurors on any possible retrial from being tainted. The Wikipedia link in the OP notes that:
In March 2018, it became known that the United States Department of
Justice has reopened the case and is looking into a possible civil
rights violation by Brailsford.
Finding jurors in that case may be somewhat more difficult as a result, although the more remote time period and the larger jury pool area of a federal court, may make it possible to find jurors unfamiliar with this video. If not, the defendant may move to change the venue of the case to a court less familiar with the case.
You are incorrect in your belief that the video evidence was not used in the case.
The fact that the video was "under seal" doesn't imply that it wasn't used at trial. This just means that it can't be released to anyone other than the prosecution team and the defense team other than at trial and that it can't be released by anyone present at the trial until the case is no longer under seal.
In fact, the full video was the highlight of the trial. According to NBC news coverage of the trial:
Last year's fatal shooting of a Texas man by Arizona police was caught
on body-camera video and showed him sobbing with his hands up, begging
for his life in his final moments.
Jurors who watched the full video showing Daniel Shaver's death agreed
Thursday to acquit former Mesa officer Philip Mitchell Brailsford of
murder, ending an emotional six-week trial.