The sleeper serves mainly to keep the wearer warm at night, even in the absence of blankets and bed covers. The sleeper covers the entire body except for the head (except in certain cases where a hood is present) and (in most cases) hands (except in cases where a sleeper has attached mitts,mostly on infant sizes), where it is snug at the neck and wrists. The use of a zipper closure in place of or also further retains warmth by eliminating drafts. This is especially important for infants, for whom loose blankets may pose a safety hazard (including increasing the risk of ), and possibly for older children, who may still be too young to be relied upon to keep their own sleepwear or bed covers adjusted so as to prevent exposure to the air of bare skin. This is reflected in advertisements by blanket sleeper manufacturers, which often emphasize that their garments "can't be kicked off", or that "no other covers are needed". The permanently attached feet can also be a beneficial feature for children who are prone to get out of bed in the morning before their parents are awake, and are too young to be relied upon to put on or other footwear to keep their feet warm,as well as for adults who find putting on,and/or wearing socks in bed too bothersome, yet still want their feet covered when getting out of bed in the morning. Blanket sleepers without feet allow more room for growth and reduce the possibility of slipping. Also, children with larger or smaller feet find a better fit.
The blanket sleeper is designed so that it can be worn either by itself as a standalone garment, or as a second layer worn over regular pajamas or other sleepwear. The one-piece design is simple to launder and has no detachable pieces that could be individually misplaced.
Blanket sleepers are usually intended as practical garments, worn mostly by younger children and only in the home. Style and thus tend not to be important in its design, and the basic design of the typical blanket sleeper has changed little over the years.
Although widely thought of as something worn only by the very young, blanket sleepers are also sometimes worn (in decreasing order of frequency) by school-age children, teens, and even adults. (See , below.)