what in c:\ProgramData\Qlik\Sense\Apps\Search\

[korsikov]

c:\ProgramData\Qlik\Sense\Apps\Search\ what is in this folder?

and sub folder name same as app GUID. and in this folders files are created with its own structure.

what kind of files and why are they needed?

it is the work of "smart search"? Cache? Why these files are not deleted after a time?

[korsikov]

dependence found.

open app.

launching a smart search. We try to find, for example, one. and we see that the folder is created with name identical  application ID and the number of files in it, probably the structure of objects in the application.

At the same time if you delete these files, search for work continues to operate without errors and the files are not created anew.

[Clever_Anjos]

Please refer to this : Re: Qlik Sense Search folders