There are reasons why it's usually preferable to do the B&W conversion on the computer rather than in-camera.
In the days of B&W "film" photography, the photography would put a color filter on the front of the camera to alter the image by selectively deciding which colors can pass through and which colors are reduced (even though what we're going for is a Black & White image). This alters the contrast.
For example... if I shoot a photo of a "red" fire hydrant against a "blue" sky with "green" grass, then the tonal values of each item will be different if I use no filter at all... vs. using a "red" or "blue" or "green" filter on the camera.
Any color in real life that matches the filters color will be allowed to pass through the filter without being blocked at all. Those parts of the image will therefore be the brightest and resemble "white" in your black & white image. But colors which do not match will be blocked and will appear "dark" in the image. If I use "red" filter to shoot that fire hydrant, the hydrant will appear "white" and the sky and grass will be "dark" and this will really punch up the fire hydrant in front of the background.
Here are some sample images to help you get the idea. These four images are all really just the same single digital image... but processed digitally using the "color" filters.
First, the color image (so you know what we're dealing with). This is an old electrical generator.
Notice that most of the generator is painted "red".
First, I'll show you a straight black & white conversion (no filters are used)
But now look at what happens if I use a "Red" filter (and from the original color image, you know that most of the painted surfaces are "red"... this means they wont be blocked but non-red things will be somewhat blocked.)
You can certainly see how the "red" filter made all the "red" painted parts of the machine appear "white" (because a red filter lets the red light through but blocks non-red light.)
Here's a "Green" conversion.
In this image, we know that there isn't much "green" in the image. So the result is a rather dark looking image. But imagine if this was an outdoor landscape with green trees, green grasses, etc.... the "green" conversion would really brighten those areas but darken down everything else.
You can use these "color" filters when you perform black & white conversions to alter the look of the black & white image and get a very different result than a straight (non-filtered) conversion.
Lastly... If you shoot "RAW" on your camera... even if you told the camera to shoot black & white, you'll still get a color image. That's because RAW is RAW. You get all the data (and "all the data" included color info). The camera simply notes in the meta-data that you requested a B&W conversion so that computer software might know to auto-apply it.
