I think the main point of adding hidden things boils down to bringing extra challenge to those who want it. This is my opinion (and I m sure there s lots of game designers out there who can add more into my answer). I think hidden objectives are a good thing compared to some alternatives.
Think about the following example.
Imagine some game that is really challenging. Imagine that it is so difficult that most of the players can t solve the first level. The game developers decide to lower the difficulty level by decreasing the number of enemies, and suddenly the game is boring for those hard core gamers that enjoyed it in the first place - it is too easy for them. Now it is okay for the masses, but there is also a group of very unhappy gamers. Putting easy , normal and tough difficulty levels could be possible but it might turn out quite dull when you have less lives and ammo in easier levels.
But then Joe Designer gets it. He decides that they will have certain difficulty level instead of easy or tough, and he puts extra challenge (hidden objectives) in the game. Now the game might be easy to solve in some parts, but there would be alternative routes (dangerous ones) that would be more challenging. There could also be additional items that are only given if certain requirements are met (for example: player would need to collect all the gold keys in the levels to get access to mine of the dwarves).
Hidden objectives can give extra incentive for players to try to beat levels. That is one reason for having them. Other reasons might be about the story line (like reveal additional information about the reasons behind the historical conflict) or perhaps they could give hints (or aid) in the upcoming levels.
Why There Are Hidden Objectives In Games?
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