Bifocal lenses are divided into two parts to accommodate a combination of any two different prescriptions into one lens. One area of the lens will have one prescription (usually distance) and a segment in the lower half will have the other (usually near vision).
Rather than having two pairs of single-vision glasses, bifocal lenses can be a more convenient solution as they combine all your vision needs into one pair of glasses.
For example, if you work in an office you may want to have intermediate vision in the top of the lens for meetings or presentations, and near vision in the segment for looking at your computer screen.