To answer your bonus question, the general answer is no, you don't need to set variables to "Nothing" in short .VBS scripts like yours, that get called by Wscript or Cscript.
The reason you might do this in the middle of a longer script is to release memory back to the operating system that VB would otherwise have been holding. These days when 8GB of RAM is typical and 16GB+ relatively common, this is unlikely to produce any measurable impact, even on a huge script that has several megabytes in a single variable. At this point it's kind of a hold-over from the days where you might have been working in 1MB or 2MB of RAM.
You're correct, the moment your .VBS script completes, all of your variables get destroyed and the memory is reclaimed anyway. Setting variables to "Nothing" simply speeds up that process, and allows you to do it in the middle of a script.