Follow the money. From the WSJ:
Campaign finance rules create an incentive for Mr. Biden to stay in the race through the Democratic National Convention in August. At that point, but not before, Mr. Biden would be able to transfer his campaign’s anticipated $100 million war chest to Vice President Kamala Harris, assuming that she, too, is still on the ticket.
If Mr. Biden drops out before the Democratic Party formally makes him its nominee, then Federal Election Commission rules dictate that no more than $2,000 of any campaign funds that he raised may be transferred to any other candidate, including Ms. Harris. The Federal Election Campaign Act governs what a presidential campaign may do with “excess campaign funds,” which is what the money left in the Biden for President campaign will legally be considered if he is no longer a candidate. Those excess funds may be contributed in an unlimited amount to the Democratic National Committee or an independent expenditure committee. Presidential campaigns may also contribute such funds to other federal campaigns, subject to contribution limits, which are $2,000 per election.
In short: Before the nomination officially goes to Mr. Biden, his campaign is limited to donating $2,000 to the Democratic nominee, whether that new standard bearer is Gavin Newsom, Gretchen Whitmer or Ms. Harris.