I’m pretty sure Legal Eagle covered this (at least partially). There are laws in place that prevent candidates from accessing funds that originally belonged to another candidate. This is a fraud prevention method to make sure that someone doesn’t join a race then drop out purely to dump their funds into a candidate of their choice. A very wealthy person (billionaire?) shouldn’t run for office with the intent of dropping out. There are very strict laws that regulate campaign finance. There are limits on how much money can be donated to a campaign, and money donated to Biden would be ineligible to be given to Harris.

That being said, the money will go towards Harris, but it may go in a roundabout way. The money may be put towards the DNC and their advertising budget which could go to support any DNC candidate. However, since she is the front runner, the money will be spent towards her. Seems like filling suit is just for show, imagine that.