Save color ink without hunting for the setting each time
Setting grayscale as the default at the printer level -- rather than inside one specific app -- means every program that sends a print job to that printer automatically prints in black-and-white, so you don't have to remember to change it each time.
'Grayscale' and 'black ink only' aren't always the same thing
Grayscale mode blends your printer's cyan, magenta, yellow, and black ink to produce shades of gray, which some printers still do even when 'black and white' is selected. If you specifically want to conserve color ink, look for a setting labeled 'black ink only' or 'true black' in your printer's advanced properties -- it isn't offered on every model, but it avoids touching the color cartridges entirely.
Frequently Asked Questions
If I set it to black-and-white, will pages with photos still print in color?
No -- once grayscale or black-and-white is selected, the whole document prints in shades of gray, with no color anywhere on the page.
Does the setting look the same on every printer?
Not exactly. The wording -- grayscale, black-and-white, or monochrome -- and where it's located in the properties menu can vary a bit by printer manufacturer and model.