Hidden bosses are a weird part of video games. Usually, a hidden boss is a sort of secret reward for the player - offering them elaborate loot and considerable bragging rights for finding (and beating) an enemy that many would never even encounter.
However, sometimes this isn't the purpose of a hidden boss whatsoever.
Because there are plenty of bosses that are obscured so intensely that it's clear the intent isn't to make you work for it, but rather to legitimately hide them from you. Wilder yet, there are plenty of games that have cut bosses from the game itself, only for hardy nerds to code the boss back into the game itself, letting you get involved in fights that developers legitimately didn't intend for you to access.
It's unusual that you fight a boss that genuinely feels like you weren't supposed to find it - but it's also profoundly rewarding, especially in renowned games such as Dark Souls or Undertale.
Although some require you to get deep into the game's code in order to patch in said bosses, the knowledge you are having a super rare gaming experience is unique - and exciting - enough that it's often worth it in the end.