The arguments we have seen supporting using magnets around a fuel line usually say that it helps combustion, decreases fuel particle size, increases fuel burning efficiency, will save the owner money, or something similar.
There are a lot of articles out there that deal with this, either for or against. Not a lot of scientific information that is accurate. The end result is usually a back-and-forth in the article comments section about why/how/why-not.
There is one thing almost all of these articles ignore. The fuel ALREADY PASSES THROUGH A MAGNET ON YOUR FACTORY VEHICLE.
The fuel pump uses a DC motor to pump fuel from the tank in almost all fuel injection engines. Whether a boost pump or a main pump, the fuel passes THROUGH the motor and past the motor's magnets.
For a better explanation of this and a visual, see here
Perhaps some could argue that these magnets aren't strong enough, or are permanent magnets rather than electromagents... but if electromagnets are required (which require energy to operate), most possible efficiency gains are lost. These permanent magnets are also larger than pretty much any of the sold 'magnetic treatment devices'.
So does magentic fuel treatment work? Well, it's a moot point. It's probably already on your vehicle for better or worse. Don't waste money on a duplicate system.