Mostly, the PEN-F’s black-and-white mode is an in-camera conversion/style setting rather than a unique capture mode.

Key differences:

• On a mirrorless camera, you can see the scene in black and white in the EVF/LCD while composing, which can help you notice tonal relationships and compose differently.
• If you shoot JPEG, the camera applies its own B&W rendering, tone curve, and processing. That can look good straight out of camera, but it’s less flexible afterward.
• If you shoot RAW, the underlying RAW data usually remains in color; the B&W setting is mainly metadata or a preview that some RAW editors can use as a starting point.

Compared with converting later in software:

• Converting from RAW in post usually gives you the most control and the best chance to optimize tones.
• Converting a color JPEG later is generally less ideal than either in-camera B&W JPEG or a RAW conversion, because the JPEG already has white balance, tone, and compression baked in.

So: not a gimmick, but not magic either. Its main benefits are B&W preview/composition and a convenient in-camera JPEG look. If you want flexibility, shoot RAW+JPEG.