You use imaginable after a superlative such as 'best' or 'worst' to emphasize that something is extreme in some way.
their imprisonment under some of the most horrible circumstances imaginable.
他们在能想像得出的最恐怖的环境里蹲监狱。
You use imaginable after a word like 'every' or 'all' to emphasize that you are talking about all the possible examples of something. You use imaginable after 'no' to emphasize that something does not have the quality mentioned.
Parents encourage every activity imaginable.
父母鼓励孩子参与一切可能的活动。
a place of no imaginable strategic value.
一个毫无战略价值的地方。