A mixed environment can be physical clients together with virtual desktops. It can also be a physical and terminal server/Citrix XenApp environments, or any combinations of these. One benefit with ThinApp is the possibility to have one single package supporting all your different environments. The same package can run on physical clients as well as on VDI desktops or Citrix environments. Creating packages supporting many different environments is of course harder than creating a package only supporting one environment.
ThinApp has some parameters you can change dynamically. The ability to change the sandbox location without rebuilding your package is the most useful one. Let's say your physical clients must store the sandbox in the %AppData%\Thinstall folder, but your XenApp environment requires the sandboxes to be stored on a network share. In this case you can still create the package with the default location of the sandbox. On your Citrix servers you...