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A little more detail: when we designed OmniDiskSweeper, we wanted to make a tool that lets the user quickly find out where the space on their hard drive is going and then make decisions about what items they want to keep and which they want to delete to free up space.

One of the reason why OmniDiskSweeper answers some questions (What files are taking up all the room on my hard drive?), while not answering others (What will happen if I delete this file from my hard drive?) is because we often don't know the answer to the latter question. Knowing which files are used for which purposes is much, much harder - it would require knowledge about Mac OS X, about various applications, and information about the various modifications to the above that users could make.

In general, we just try to steer away from tricking the user into thinking that we know more than we actually do. (As opposed to other applications, which will tell you "deleting this won't do any harm" - and then turn out to not be correct.)

The approach I generally recommend to folks who ask this question is that you're safest if you only delete files that you're sure you don't need any more. Admittedly, you're undoubtedly leaving files on the hard drive that you could delete without harm if you do this, but it's also the only advice I can give you that couldn't cause you to accidentally delete something important.