What are hanger bolts for?

Answer number one:

The pointy end screws into wood (use a spanner on the squared section in the middle), then you fasten something to the blunt end with an ordinary nut.

In this example, the bolt is being used to stiffen the corner of a wooden frame by pulling the metal plate into the timber joint.

Answer number two:

They are part of the gear system of a bicycle.

Hangar bolts (that resemble the threaded portion of a machine screw attached in-line to the threaded portion of a wood screw or lag bolt with no heads in evidence) are most commonly used in the light commercial and residential construction industry to attach wall-mounted porcelain plumbing fixtures (i.e. toilets, sinks, and urinals) to wooden framing. The lag or screw part is driven into the wooden framing using a pair of vice-grips or channel-locks and then the fixture is hung on the bolts through pre-cast holes and secured with nuts and washers.

2 things here: 1.What you are hanging. 2. What you are securing the said hanging item into or on.

"Hanger bolts are ideal for overhead applications, such as suspending electrical wiring or sheet metal from wooden structures." answer from google. It's wood threaded on one side and metal (nut) threaded on the other side such that you can drive it into the wood, then bolt something to the wood at varying heights. http://www.usifaz.com/Web%20Site%20Pics/Screws/Hanger%20Bolt.gif