When you convert the BMP or RAW to PNP a fair amount of file information is discarded. NO PROGRAMS that I've heard of manipulate images in any significant way in any of the common formats. They all convert to the program's "native format," so when you save from the editing program's "native" file back to PNP there's an additional discard of info. You can save and retrieve a pnp without loss, but manipulating it and making a new pnp from the edited file is about the same as saving in a low compression jpg, so far as I can tell.
You can copy and paste, and in some uses you can "rescale" to different display sizes. The stored file can be brought back to the "open" form for display; but beyond that there's marginal benefit that I can see.
Many of the "lossless" programs use "indexed color" and there's no standard (or consistently used) set of index values for most of them, and if a pnp (or tif) file is moved between programs that use a different set of index colors, there's additional loss - or sloppy display.
The way most people misuse their files is the cause of most image degradation, but there are advantages (and disadvantages) to any format you want to pick. PNP is fine. I just don't happen to use it much.
TIFF is probably the most misused. You can specify the index values in a tif file, but a given program will assume its own set if the spec is omitted. There are at least a dozen different "tif flavors" and the standard lets you "make your own" so not all tif files can be read by all programs that use another tif - but theoretically they're all "lossless" just like pnp in compression and decompression.
The key is to pick one that works for you, but I'd recommend that you avoid insisting that your choice is the only good one.
Lossless compression just means that the file you save will come back as the same file you saved. It doesn't mean that it will come back as the same original file you made that file from.