As a landscape photographer my use of black and white as a medium is often applied in situations where colour is not the priority, but rather where light, composition, shape, texture, motion and secondary elements such as shadows and silhouettes become my primary focus.
I am especially drawn to soft colour and light as a starting point for most of my compositions but the opportunity for a strong black and white photograph often presents itself, at least in landscape photography, during less than ideal lighting conditions, often where harsh or mottled light is present. Nevertheless, this is where a "plan B" approach will provide the opportunity to capture and convey strong contrasts and tones, offering a different and perhaps more challenging, but equally as satisfying perspective for the viewer. Removing colour from the equation draws the viewer's focus into these other important compositional elements.
In an age of colour accurate digital sensors there is still something perennial about black and white photography. The digital age also makes black and white photography more accessible than ever and the ability to pre-visualise the scene in black and white will help you attain the best possible results at the time of conversion. Switching the digital camera's LCD (or in some cases even the viewfinder) creative effect mode to a black and white aids the process of better understanding whether the scene suits a black and white final image. However, I highly recommend to always shoot in RAW to retain as much detail and creative control in post processing.
Situations that provide excellent alternatives to colour landscape photography are the availability of side or back lighting, misty/foggy conditions, long exposures and minimalist compositions.
A strong defined side light helps to draw the viewer's attention to detail as it will help lift shapes and textures. Strong side lighting creates side shadows and powerful contrasts that work particularly well for portraiture and architectural photography by pulling the viewer's eyes through the darker shadow areas and along to the lighter subject details lifted by this kind of light. It's the strong contrasts between light and shadow areas that create definition of shape and texture within the frame.