good question. Because there IS no question that Biden and his posse are a bit angry about being forced out. And it would certainly make old Joe smile to see Kamala go down in flames. He very much thought that HE was the only one who could beat Trump. Delusional, but true.
Earlier this month, we also learned that Biden loyalists are still harboring resentment over his ouster from the race and the way he was insulted at the Democratic National Convention. There was even talk about some kind of revenge.
Biden is playing the role of a loyal Democrat trying to help Kamala get elected. But the reality is that he is bitter about being pushed out of the race against his wishes and wants his legacy to be that he was the only Democrat capable of defeating Trump. By joining Kamala on the campaign trail and strengthening the link between the two of them, he might be trying to solidify that narrative.