It made sense precisely because she wasn’t ever going to win a primary on her own. Biden acted like Trump here. Trump surrounds himself with people who have zero future without him. His cabinet is filled with profoundly unqualified people whose only real qualification is unwavering loyalty to Trump. This is a classic move of authoritarian leaders, as their advisors then become completely dependent on them. Once Trump is out of office, Hegseth is never going to have a role in government again. (Unless some other authoritarian president is seeking a loyal lackey.)

Biden did a less extreme version of this with Kamala. Kamala certainly wasn’t objectively unqualified for her job like Hegseth is, but she also could never had obtained a leadership position in the White House via her own merits.