So that they can be printed clearly and crisply.
DPI or "Dots per Inch" is the measurement used in the publishing industry to determine how sharp an image is. The more dots the higher the quality of the print (more sharpness and detail).
To make sure you're providing 'useful' images for print they should be 300dpi or higher. Image dimensions are also important. Obviously, an image that is 300dpi but 2cm big is not useful for most people! Think of the height and width of most magazine pages as an example of the dimensions you should have.
For online media 72 dpi is sufficient, but remember - when it comes to SOURCE images, bigger is better. You can go down in size without losing quality, but you can't make things larger without compromising your photos.
Photo credit: Sabine Ducasse