I love all the conspiracy theories on twitter on why Barack, Schumer, Jeffries, Pelosi haven't endorsed Harris yet but Clinton, Kerry, and other current senators/house members have.
Clintons and Kerry are retired now. Their endorsement is a message to the older voters of the party, the blue dogs who will have a higher turnout anyways.
Schumer is senate majority leader, Jeffries is House Minority leader, and Pelosi is a 2x Speaker of the House, and she's still actively serving and still has some political clout.
The obvious answer (I guess it's not obvious to anyone who doesn't understand political optics and strategy), is Obama is the biggest endorsement Harris could get, bigger than Biden's. The DNC Convention is this week. Party Conventions are just as much political theatrics as anything else. THis isn't anything new. Any sane President would endorse their vice president. Biden endorsing Harris isn't breaking news. Trump is the only person who would leave his vice president out to dry, look at what he did to Pence lol. Considering JD Vance was a huge Never Trumper, it wouldn't surprise me if things go sour that Trump throws him under the bus almost immediately.
2008 the convention was used to unite the party. The big story was getting the Clintons to unite the party and get behind Barack. That's how it played out.
Obama is still very popular amongst millenials and young Dem voters, many who weren't even old enough to vote for him back then but are of voting age now. Now that Barack isn't going to give the usual stump speech at the convention for Biden, his speech will probably have millions more viewers tuned in to see how he endorses Kamala. Again, it's political theatrics, and it's something both parties use the conventions for. To tell a story, just like WWE does in wrestling.