The key is in how the Democrats’ delegate math works. The rules require candidates to receive at least 15 percent of the vote, typically, to win delegates statewide or at the district-level.
Buttigieg was projected to get under 15 percent in the vast majority of states and districts on Super Tuesday. Thus, his votes were essentially wasted. Redistributing his votes to other candidates will help them to meet the 15 percent threshold, however. In particular, Sen. Elizabeth Warren and former New York Mayor Michael Bloomberg were both close to the 15 percent line in many states or districts. So even an extra percentage point or two would help them get over that line in more places. For instance, both Bloomberg and Warren were projected to finish with an average of 14 percent of the vote in California before Buttigieg’s dropout. Now, they’re forecasted for 16 percent instead.
Biden was also projected to finish under 15 percent in some states and districts — so Buttigieg’s dropout helps him out also in a few places. Biden went from a projected 14 percent of the vote to 16 percent in Minnesota, for example.
Conversely, Sanders was already projected to get 15 percent almost everywhere. So although he will pick up a few Buttigieg voters, they don’t necessarily translate to more delegates.