nospam
the changes are rendered on the fly and may be cached to disk (which is the preview file you're talking about). the latter is optional.

Sandman
It's not optional. All images in LR are always rendered as previews. They are kept inside your LR library.

nospam
it is optional.

Sandman
Not in LR. They are a mandatory part of the image import and workflow process.

nospam
previews are for a responsive user interface.

Sandman
Of course.

nospam
they are not required as part of a non-destructive workflow.

Sandman
They are, however, required in Lightroom's workflow, which is non-destructive.

nospam
it is not a pixel editor.

Sandman
Sure it is, only in another sense than old Photoshop. Every single adjustment you make in LR are applied to the pixels and saved to disk as a preview image. Difference is that the original file is always kept intact so every step is fully reversible.

nospam
nope. lightroom is a parametric editor, not a pixel editor.

Sandman
It can be, and is, both. The parameters are applied to the pixels as they are made, in the preview file. Also, of course, in any exported files.

In fact - using smart filters in Photoshop is *less* of a pixel editor than LR these days, because the pixels are never touched, it's all kept in RAM and the resulting image is never saved to disk until you export/save it. Also, fully reversible of course. :)

nospam
photoshop is a pixel editor but can be used in a non-destructive manner. that doesn't make it a parametric editor.

Sandman
Using it as I described above makes it one, yes.