The reason Compass Carburettor had invested in Muscle Lantern's growth was that he was an asset in training students. He already was one before, for he trained the physiques of Heroes to give them fast growth.
But now, with his upgraded abilities, he was able to give them physical, ability, and mental growth. This would allow the students to tap into their potential faster and experience the most optimal results for their training.
Usually, trying to improve oneself was very difficult as it was hard for everyone to picture a better version of themselves. Not only was it mentally draining, but egotistically speaking, it was hard.
In stories, the main characters would get into a training arc where they would prepare for exactly this so that they could picture a mildly stronger version of themselves and grow to defeat them.
