why use 'optimized media'

If you find that you can use your native footage OK stick with it.

If it doesn't work well when doing simple editing you need to convert ("optimise") to ProRes422 which is more suited to editing.

If you are doing very complex editing involving numerous effects and tracks you may find you are dropping frames. To prevent this you should use proxy media.

The Reasons: Highly compressed footage like AVCHD is not good for editing as it puts a tremendous strain on the processor apart from other potential problems. The processor has to decode every single frame.

Converting it to ProRes makes huge files but the processor can whip through them effortlessly.

Think of it as being a bit like the gears on a car or bicycle. When you are going up a hill, a high gear with the enguine/feet moving slowly is incredibly difficult to use but a low gear with the engine/feet racing around is much easier.

The large ProRes files are a bit like the fast turning engine in a low gear . . . they put less strain on your computer . . . even though they take up a lot of space.

I've never used proxy material and most people never need to.

So unless you are having problems like I outlined above, you can forget about it.

It is probably larger because it is much less compressed than the original and therefore easier to use.

However, it is of very low quality and in the end has to be converted back to a high quality when you export your project.

The main rule to apply is, "does it work well for me?"

If it does, stick with it.