There is really only one reason for the existence of hair straighteners, and that is, in a word, fashion.  If you have naturally curly hair and want to straighten it out, the only way to do it is by applying heat.  Basically, straighteners work by breaking down the hydrogen bonds in the cortex of the hair that cause it to bend and curl.  These bonds will reform given a warm, humid atmosphere, thus the frequent use of straighteners by curly-haired women who want the straight look.

One important fact to note is that heat in excess of about 100 degrees Fahrenheit  (37C), applied directly to your hair, has the potential to cause damage.  The greater the heat and the more often it’s applied, the greater the risk of damage.  For thick, curly hair, the temperature needed to straighten it effectively is around 200 degrees F.

Those who frequently dye their hair or use perms on a regular basis, or those whose hair has already been damaged by too much sun, should be especially careful about the hair-straightening process.  The loss of moisture due to direct heat or chemicals can be aggravated by straightening and result in brittle, lifeless hair if you’re not very careful.

The good news is that the recent technology in the hair-straightening department has vastly improved over the 1960’s ironing board method.  All of the better products are designed to help your hair retain moisture and even increase its health and shine.  The top end products use infrared radiation from ceramic plates with an ionic process that breaks down water molecules and actually makes them easier for the hair to absorb. 

The bottom line is that the degree and frequency with which you heat up your hair is directly related to the damage you may do, but modern products should do no damage at all if used correctly.