Bonus Minigames (or practice minigames) were originally added to Darkhouse in update 0.1, with 5 stages unlocked when the player reaches each of the 5 locked rooms in the main game. They were intended for practicing your skills, because at the time Darkhouse only had the Hard difficulty mode, and you would always start from the very beginning. It provided a way to practice for the later levels instead of being stuck behind earlier levels at the start of each game.
But now that Normal and Easy mode have been added, with saved checkpoints keeping your progress, there's no point to playing Bonus Minigames to prepare for the main game. While the main game has set goals, Bonus Minigames are endless and unrewarding. There's not even a point to playing them for fun, because you could play the main game for the same reason and still accomplish something, like beating a boss. There was a bug when medals were first released that allowed players to gain medals for large amounts of score and defeated enemies during the minigames. Those medals were intended to be obtained in the main game only, and Darkhouse was updated to fix the issue. I did have an idea to add new medals for Bonus Minigames, but since the minigames don't really have a purpose, I didn't think I should add medals for them.
I think it's probably best that Bonus Minigames are removed from Darkhouse. I really hate to remove content from a game, especially the main addition of a major update. But these minigames interfere with the idea of Darkhouse's linear progression, and there just doesn't seem to be any reason for them to exist. The focus of Darkhouse should be to complete the game, and to reward those who do so, there is a secret minigame afterwards, a kind of epilogue for Darkhouse. It provides endless gameplay and new challenges, and it also allows you to set high scores to the scoreboard, meaning there is no limit to the highest score achievable in the game. Bonus minigames are far less purposeful and interesting than this secret minigame, and far less rewarding.
I thought I would give a heads up before I remove the Bonus Minigames, and if you think they should stay then I'd like to know why, and how they could instead be improved. But to me, there just doesn't seem to be a reason for anyone to play them, while there's so much more in Darkhouse waiting to be discovered.