Foot orthoses have become a multi-billion dollar industry in the U.S. The most common reason is the diagnosis of "over" or "excessive" pronation. As Prime for Life (the book) points out, all feet normally pronate, which is very roughly a rolling in of the arch. Almost all studies examining the ability of ped-orthists, physical therapists and podiatrists to assess the foot's motion and position either standing or in motion have consistently shown very poor inter-rater reliability. This means that although in some studies individual examiners are fairly consistent in their own findings about individual feet, there is almost no agreement between clinicians on what they find! They can't even reliably agree whether the foot is abnormally pronating! And this was found in experienced podiatrists! (see Van Gheluwe et al, 2002 on Evidence page in Foot chapter on Prime for LIfe website)
     Even the idea of "over pronating" has not been quantified or explained well in the medical literature. The typical method for making custom orthoses, which typically cost $300 to $1000 a pair, is the "subtalar neutral" method. This method was developed by a Podiatrist in 1966 based on a prior study, which measured foot motion in two subjects. The podiatrist, Merton L. Root, DPM, was ahead of his time and provided a conceptual way to improve foot function with orthoses. However, he misinterpreted the original study (see Wright, 1964 under "Evidence" page in foot literature on Prime for Life website), presuming that when one is walking with the body right over the foot that the foot on the floor is neither pronating nor supinating. He called this "subtalar neutral" and devised methods of placing the foot in this position for casting orthoses.  All subsequent larger, better quality studies have shown, however, that the foot is to some degree pronating in normal feet the entire time it is on the ground. The foot never, in normal feet, attains the subtalar neutral position on the ground. Rather, they have consistently shown that it is always pronated on the ground. So orthotics, which are designed to place your foot in this neutral position, essentially attempt to place your foot in a position that is only attained after the foot leaves the ground swinging in space.
    The good news is that other studies have shown that orthoses don't significantly alter pronation and supination anyway (See all studies by BM Nigg on Evidence page on Prime for Life website under Foot and Ankle chapter). We do not really understand why they do help people. It may alter proprioception or sensation and /or muscle activation patterns. Or it may simply disperse forces over a larger surface area decreasing pressures. Nigg has shown that comfort may be the most important factor dictating success of orthoses - not some mythical ideal foot position that is never attained. Some research has shown that inexpensive over-the-counter orthoses and arch supports may be as helpful as custom ones that cost a fortune. Try the ones recommended in Prime for Life.
    If you have unusually shaped feet, such as very wide feet or deformities from arthritis, then custom ones can really help improve comfort. Be wary of shoe salesmen who purport to be able to accurately diagnose foot types. If podiatrists don't have good inter-rater reliability assessing feet, don't expect someone with very little training to be good at it. However, that said, some shoe stores are getting better at fitting shoes to feet. While no one can reliably tell what a foot is doing inside the shoe, rough "gestimations" may help. Wear shoes inside for at least 20 minutes (on a treadmill or in a mall) to see if they are comfortable.  That tells the most.  When your feet are happy, so are you.   For significant foot problems see a physical therapist who specializes in feet or a podiatrist.