Is the energy absorber of a gear kit for via ferratas for just one use? I mean, one fall and it has to be replaced?
In the product description, there is often mention of it being "tearable."
There are such things as reusable energy-absorbers, they use rope and a braking plate to absorb the energy of a fall. Stiched webbing energy-absorbers are only intended to catch one fall, but that fall typically has to exert at least 2.5kN (250kg) on the device before it will start to tear.
If the threads in the energy absorber tear, then yes, it needs to be replaced, if you do not replace it, then it will not absorb as much or any shock at all the next time you take a fall and could fail.
Tearing is how they absorb shock forces, the lanyard is actually rather long, but it's folded over itself and stitched in a way that when you take a fall, the stitches will take the force and tear. This relieves shock-loading to the anchors and your body.
It's important to inspect your lanyard for signs of damage before each use and after any fall. You should retire your system if the energy-absorbtion lanyard:
is cut, worn, melted or abraded
stitching is broken or shows signs of wear
has any amount of energy-absorbtion webbing separated
Basically, if your lanyard hasn't gotten any longer after you take a small fall, then you probably haven't exerted enough force to cause any tears or separation, but always inspect your gear carefully to be sure, the reason you use this stuff is so it can potentially save your life in the event of a fall. Whenever you have any doubts about your equipment, you should always choose to retire it (destroy it so no one else can use it) and replace it.
Reusable energy absorbers, more commonly known as energy absorbing plates, or ferrata breaking plates, have been around for a while. They're still in use, but they've largely been replaced by the webbing absorbers because the webbing absorbers are lighter, and don't have a tail of rope that needs to be managed somehow.