Like past runners-up, Mr. Sanders will want a prime-time speaking slot at the convention itself, as well as prominent speaking opportunities for some of his supporters. He may wish to have some of his aides installed on Mrs. Clinton’s campaign or on the Democratic National Committee. It is likely that he would want to be consulted on Mrs. Clinton’s choice for a running mate. (Chances are low that she would pick Mr. Sanders himself, but that is what negotiations are for.)

He may even seek a full roll-call vote, in which his hundreds of delegates would seek to nominate him from the floor in Mrs. Clinton’s place.

But he lost. Why would Democrats give him anything?

Because of what he can give them: a vast list of potential supporters whom Mrs. Clinton and other Democrats could mine for votes and money. Young Democrats, in particular, have been drawn to Mr. Sanders this election cycle. His supporters are particularly important to Mrs. Clinton. Recent surveys suggest that anywhere from half to almost three-quarters of those in favor of Mr. Sanders are willing to vote for Mrs. Clinton. She does not need all of them to win the White House. But the more Sanders supporters she can persuade to come to her side, the better her chances will be against Donald J. Trump.

Why would Clinton have a problem with any of that?

Acceding to too many demands may risk tying Mrs. Clinton to positions she does not agree with. There may be some platform requests that could hurt her in a general election. (Proposing a ban on fracking, for example, would be unpopular in states like Ohio and Pennsylvania, where the drilling technique generates lots of jobs and tax revenue.) Mr. Sanders’s representatives have also signaled that they will seek to alter the party’s platform planks on Israel to better reflect the views of Democrats critical of the country’s government, a debate that could provoke a bitter fight within the party. A roll-call vote would show up Mrs. Clinton at a time when she would want to project strength.

Is there any downside for Sanders?

By one accounting, very little. Mr. Sanders, 74, is unlikely to mount another bid for president. His Senate seat seems safe. Senate Democrats, itching to retake the chamber this fall, can hardly afford to shun him. He seems to welcome, rather than fear, the scorn of party leaders. And of course, he isn’t even a Democrat. He is an independent. (He caucuses with the Democrats and is counted as one in the arithmetic for which party controls the chamber.)