<p>Lately I noticed that there is a new type of nozzles (called airbrush nozzles?!?) available; typically found on those online overseas vendor sites. The nozzles are advertised for usage in E3D hardware, but are not found amongst the <a href="https://e3d-online.com/nozzles-for-3d-printer" rel="noreferrer">E3D genuine nozzles</a>.</p>

<p>These nozzles look like this:</p>

<p><a href="https://i.stack.imgur.com/OH4O2.png" rel="noreferrer"><img src="https://i.stack.imgur.com/OH4O2.png" alt="Airbrush nozzle"></a></p>

<p></p>

<p>What are the basic physics principles or what is the engineering relevance for application of airbrush nozzles? (Gimmick or actual product improvement?)</p>

They are used for nonplaner 3d printing.
https://all3dp.com/4/nonplanar-3d-printing-gives-the-smoothest-top-layers/
The gist of it is, that You can achieve smoother, curvy top layers that are more true to form, buy using a nozzle like the one you mentioned. Also, they can be used to print multiple objects at the same time, where is faster to print 10 to 20 layers of one object then switch to the next object on the build plate, rather than having to print each layer of each object sequentially. This reduces build time, stringing, voids and the potential knocking off the part by the nozzle (as it moves to print the next object, since that object may have shrunk/expanded since the nozzle last visited it).