Foot orthotics or supports are designed to correct biomechanical issues of the foot and to strengthen the biomechanics of the foot. There are lots of various kinds of foot orthotics and methods of making them for doctors to use and select from. A lot of people need and do benefit from customizable foot orthoses while others might be helped by the easier and less expensive mass-produced foot supports. Which one is best for each individual depends on what exactly is causing the problems and what the treating clinician advises is the best for that individual. All different foot orthoses have got unique design features and a good clinician will attempt and match those up to the requirements of the individual. If this is not done properly, then there may not be any improvement in the symptoms.
Foot orthotics are not without their issues. If they are not the right choice for the individual, they'll likely will not work. You are also restricted by what footwear you can use, in that you need to be able to fit them into the shoe and there may not be adequate room. There are also types of shoes, just like the flip flops which they simply can't be used in. This tends to cause an issue in warmer climates which make closed in shoes meant for foot orthotics uncomfortable. There are alternatives, for example, the Archies which have a foot support built in. The design of the arch supports in these flip flops resemble what you would get in an over-the-counter arch support, but most likely not as good as what you would get from a custom made foot support. Using these do give you a practical option to foot supports, depending on where you can wear them and the character of the biomechanical issue. If you do need foot orthotics there isn't any harm in trying these kinds of choices or raising it with your treating podiatrist to determine what they think.