Why in he'll is Biden and Garland having the DOJ fight this? Not for the need to politically embarrass Trump (almost redundant at this point), but because any good government advocate must realize that Trump and Barr ran the DOJ like Trump's personal mob family firm. The misuse and abuse that ran rampant the last four years must be fully revealed to the public!
Just spit-balling here but theoretically speaking, the memo might potentially suggest criminality that's tied to ongoing investigations & pending cases (e.g., Giuliani's, etc.), meaning the release of the memo right now could result in a situation wherein the DOJ would need to be accelerate said cases & potentially fast-track indictments at a faster rate that they were otherwise prepared for. Who knows, though?