Hotboxes are used saturate your ski bases with wax. By using a temperature much lower than the melting point of the ski base, hotboxes are safer for your skis than using a wax iron for base saturation.

Why do you want your bases saturated with wax? Essentially, wax conditions the base to have a certain hardness and water repellency. Having wax in the base actually allows the base to absorb more wax.

Wax also protects the base from oxidizing. An oxidized base does not absorb wax well, and a naked base is a slow base. As you ski, wax wears out of the base. Having more wax in the base allows you to ski longer before you have to rewax to protect your bases.

Notice I said rewax to "protect" your bases, to not to increase the performance of your skis. Hotbox may saturate your bases with wax, but you still need one of more layers of wax for the day's conditions.

 

 

Criteria Hotbox Waxing Iron Winner
Gentleness
to skis		 The optimal hotbox temperature is a cozy 50-55ºC, well below the P-Tex melting point of 85ºC.
  It's not necessary for the wax to be in a liquid state to be absorbed. Wax flows and bonds at temperatures below it's melting point. 		You must melt the wax with an iron to get it into your skis. You can easily crank up your iron past the point where P-Tex begins to melt, all the way to 135ºC when P-Tex goes into full meltdown.
   Irons - even good ones - also tend to spike to high temperatures.		 Hotbox
Deep wax impregnation Mark Waechter of NordicTune tested wax absorption by weighing skis before and after waxing. His findings? Hotboxed skis absorbed 2-3 times more wax than the iron method.
  Toko came up with similar results, comparing their Thermo Bag to ironing.		 Eternity - the time it take to iron as much wax into your bases as you can with a hotbox. Hotbox
Speed of waxing The longer the skis are in the box, the better: 45 minutes minimum, 3 hours best, overnight OK.
  Then you may still have to iron in a final race layer...		 A couple minutes per ski, including putting the skis on to your wax bench. Iron
Portability Homemade hotboxes are not very portable - heavy and bulky. Some of us don't even have room in our home for a hotbox!
  Of course, you could spend $6,000+ on a Toko Thermo Bag if you really wanted to...  		Throw the iron into your wax box, toss it into your car. You can use it anywhere. Iron
Cost Building a hotbox is roughly the same cost as a high-end iron. But you need an iron anyways to put the base coat of wax on your skis before you hotbox them, and again to iron in the wax of the day. You already have an iron... Iron
Key advantages A long lasting base wax that's gentle on your bases. A shorter lasting wax job, but faster and less expensive.  
Key tips Don't let the temperature of your wax box exceed  60ºC. Keep you iron moving! The key is to melt the wax, not the base!