A black and white copier only use one toner colour – black (k). They produce monochrome copies, which is why they are called mono devices.
Mono devices are ideal for companies or departments that print lots of invoices, bills runs, worksheets or large text based documents that require no colour. If you just add a coloured logo to your documentation, it might be a good idea to have pre-printed letter headed paper in one paper trays which can be printed on with just black text to keep costs down.
If your business doesn’t need to use colour for its printing output, then a black and white copier could be the sensible choice, as they are far cheaper than a colour device, both on purchasing the hardware and service costs.