It's developing your characters skills that makes completing side quests and bonus bosses worth it. (thekoalition.com)
As part of a collaboration between Yacht Club and Inti Creates (likely due to Yacht Club publishing Gunvolt at retail in the US), Shovel Knight will be a special bonus boss in the upcoming Azure Striker Gunvolt 2. (monamiibo.com)
A whole plethora of bonus bosses, including a Game Freak employee, a rematch with the Team Skull leader and various trainers that only appear after you beat the game for the first time. (gamingreinvented.com)