Backing, sometimes called blocking is when a piece of wood in mounted between two studs or two ceiling joists. This can be done during framing of a new home or during an addition to an existing home, or during a renovation when the demolition exposes the studs and ceiling. The piece of backing or wood needs to be flush with the studs so there is a smooth surface when dry wall is applied.
The tapestry shown above is approximately 6' X 8' in size and is substantial in weight. To be able to screw the decorative hooks that hold the tapestry into backing will insure a much more stable mount than just mounting the hooks directly into drywall.
A mirror such as the one shown above is extremely heavy. Often a 1' X 4" board is mounted on the outside of the wall and screwed into a stud to insure stability. An easier solution is to have backing in the wall underneath the drywall.
A discussion about backing with your audio/video team is recommended. Where will televisions be mounted? What about stereo speakers?
Towel bars, towel rings, and towel hooks all need backing behind them, especially if you have young children. I know I have seen a towel bar pulled out of the drywall on many occasions.
Like the towel bar, I highly recommend that toilet paper holders be mounted into backing. Over time, the subtle movement applied when using toilet paper in addition to the movement applied when swapping out a roll will affect the stability of a toilet paper holder that is mounted only into drywall. A toilet paper holder mounted into backing is much more secure.
(The white tail deer trophies shown in this photo are my son's, hunted with a bow and arrow. The meat is eaten.)
A few items found mounted on a garage wall include bicycles, kayaks, and tennis racquets. Make a list of what you might hang on your garage wall and discuss backing options with your builder. You may even decide on some type of wood walls. In this particular garage there is a wainscot of beadboard with drywall above, so backing was needed behind the drywall.