Usually, a party's losing presidential nominee will still hold some sway with the next crop of candidates. Whether they mean it or not, the party's top brass will throw the former candidate's name around, extoll their many virtues, and perhaps produce some sort of "role" for them to play in the nominating process. The candidates themselves will kiss the ring, and seek endorsements or aid from defeated, but still relevant, political machines.
Think about Mitt Romney. Almost immediately after the 2012 election (right up until Trump secured the nomination) there were constant, repeated, calls for team Romney to select an heir-apparent, or to throw their own hat back into the ring. Heck, there are a few Romney die-hards who are still trying to coax him back into politics.