I saw somewhere the theory that Biden would tap Garland as AG, opening up a key DC Appeals Court seat where Biden could slot in Ketanji Brown Jackson, 49, who clerked for Breyer, who she'd supposedly be replacing. Biden has pledged to appoint a black woman to SCOTUS. And elevating KBJ to the DC Appeals Court would up her resume. Reportedly KBJ was on Obama's shortlist back in 2016.
The trick would be getting Breyer to retire int he next year or two.
Also, giving Garland a high profile gov't role can help remind folks of the Supreme Court steal the GOP engineered. Keeping the base mobilized. I don't know enough about Garland to say how he'll do as US AG, and he was portrayed as rather centrist, but he also has reason to have a bit of a grudge and disdain for GOP hyperpartisan BS.