With Nixon, Ford (AIUI) wanted the country to try to 'move on' from all the non-sense that Nixon tried to do. He was afraid that further investigations could cause more damage.
With Biden, he (AIUI) wanted to make sure that certain people (Fauci, Milley, etc) didn't have a legal sword hanging over them because they disagreed with trump. Fauci received death threats against him and his family because of his work/statements during COVID, and the government provided a security detail, which Trump has cancelled (and Fauci has to pay for himself):
