A footbed's primary purpose is to create an interface between your flexible foot and the rigid boot. In doing so, it controls the movement of your foot and ankle while skiing thus (when done properly) eliminating problems like toe bang, shin bang, ankle pain, etc. It is also the necessary requirement to have before getting shell work done (if your foot is not properly supported, any shell work is fruitless).
Now, there are a few different types of footbeds: trim to fit and moldable/custom. Typically, if you wanted the best interface between your foot and the boot, moldable/custom was the best way to go because it was adapted for your specific footshape. Trim to fits are great because they are so cheap, about 1/5 the cost.
Typical trim to fits are a good start but they are what is known as a static footbed- the do not change shape nor are they moldable. The ALINE footbed is the only trim to fit that actually is dynamic- it changes depending on your footshape and alignment needs. It is the only trim to fit worth spending money on, in my opinion.
Custom footbeds are only as good as the person making them, and this is the difficult thing- none are created equal, even from the same fitter. But when done correctly, can produce an awesome interface between your foot and the boot.
The ultimate answer is getting the right match for you- creating the best interface between your foot and the boot. Customs are usually the answer, but I've seen ALINE out perform/align people better than the nicest customs made. It just depends on what works for you.