You have to add such a vacuum layer in codes that only feature periodic setups in all 3 dimensions and even in such periodic slab calculations you probably have an interaction between the different replicas of the thin film. In Fleur we have a "film mode". Have a look at https://www.flapw.de/MaX-6.0/documentation/filmBasis/ for details. This mode lifts the periodicity condition in the dimension perpendicular to the film plane and with such a setup you don't need to add extra space in your unit cell to simulate the vacuum. There are also some quantities that are pretty easily extractable from the Fleur output in such film calculations. For example the work function.

Having said that, there also are a few Fleur calculation modes that are not compatible to the film mode. Here, I am thinking about calculations employing hybrid functionals and using Fleur in combination with the SPEX code for calculations employing the GW approximation. In such cases you can still perform periodic slab calculations.