The normal versions of this tool are simply marking aids (see page 34 of Charles Hawyard's How to Make Woodwork Tools here on Toolemera for the classic example of this type). They've always been user-made apparently.
They're used simply by laying over the edge of the workpiece you'll be cutting your tails or pins on (either one can be cut first) and then you mark your dovetail angles on the face of the board. Note that this is a wooden tool so it is for pencil marking only, use with a knife or scriber will all too quickly wear the edge and render it inaccurate.
As the version in the video has magnets it's intended as a sawing guide.
But I miss on how you use the thing.
You can see Adrian Preda testing the tool out at 4:16 ^_^
The other thing was, what does the angles have to do with dovetails?
Mainly specific angles are for different wood types (the major separation is often said to be between softwoods and hardwoods, but really it's between soft wood and hard wood more than between the two kinds of wood).
But there are also aesthetic considerations when it comes to dovetail angles. Sometimes you can simply choose the angle that you like and pay no heed to whether this is supposed to be a suitable angle for the species of wood you're working with*. Just as with the size, and spacing, of dovetails user preferences trump should-do or must-do. This isn't a modern thing, where we're going against tradition for the sake of modernity, many many old dovetails violate 'the rules' and the fact that they're still around for us to point at and go "Look at what they got away with!" shows this.
*Remember that the dovetails are only a part of the way the joint holds together, the glue locks it firmly together and prevents any movement. Compare to a finger or comb joint for example, where there is no dovetail shape advantage and yet the corners hold together just fine because the glue is doing most of the work.
Dovetails are partly for when the glue fails, the thing will still hold together, but there is no absolute reason to build joints with that consideration (same reason all tenons aren't wedged).