Is there any reason to use in-camera black-and-white or sepia effects instead of editing later?

Usually, post-processing is the better choice because it preserves maximum flexibility. Keeping the original color image lets you decide later how to convert it, and software typically gives you more control over black-and-white toning and similar effects.

In general, the camera is not doing anything inherently “better” than a computer for these digital effects. In-camera processing tends to make assumptions for you, while editing later lets you make those choices yourself.

That said, in-camera effects can still be useful:

• for quick results when you don’t want to edit later
• for immediate preview or client feedback on how a black-and-white or sepia image will look
• when shooting RAW on cameras that store the effect as metadata or processing instructions rather than permanently changing the RAW data

If your camera behaves that way, shooting RAW with an in-camera effect can be effectively lossless: you get the preview/look in camera, but still retain the full-color original for later reprocessing. A practical compromise is RAW+JPEG, so you get a ready-made version plus the untouched original.