Because garages are utilitarian spaces in most homes, many people leave them unfinished, not bothering with the expense and time necessary to finish them to the standards of the interior of the house. For reasons of aesthetics, fire safety or heat retention, others choose to finish their garage ceilings.
An unfinished garage ceiling may not be the most aesthetically pleasing part of your house, but there are advantages to leaving it this way. When the joists are left open, it is an easy and convenient place to store things such as lumber, outdoor equipment and small lawn items such as folding chairs. Leaving the joists open may also decrease the chances of having problems with squirrels in your garage attic.
Many garage ceilings are drywalled so that insulation can be added on top of them. For a heated garage, this is obviously a necessity. Even if the garage is not heated, a drywall ceiling will keep it somewhat warmer because it will trap ambient heat inside the garage. When the drywall is installed for purely utilitarian reasons, some people leave it without spackle or paint. Other people choose to spackle it in order to seal the joints, but don't prime it or apply a finish coat. Still others intend to have a finished ceiling but just never get around to it.
For the best looking garage, a finished ceiling is standard. Once the drywall is installed and spackled, it isn't that much more work to apply primer and a finish coat. This not only makes the ceiling look better, it helps to protect the drywall from damp and possible mildew. A bright white finish coat also improves visibility in the garage by reflecting ambient light into corners and off the walls. Finished ceilings can be flat, with the drywall applied to the bottoms of the joists or cathedral ceilings with the drywall attached to the inside of the roof rafters.