str() is indeed unnecessary, and the code would work fine without it. I just have it there to remind myself that I am iterating through a string and not a list, or something.
The program runs through one iteration per character in a string (that string being PARAMETER). If that character is a vowel, the program removes it from the string.
To remove a character, the program had to have checked if it was a vowel first. Meaning, it already ran an iteration. This means that a four character string with no vowels at all, would run through the same amount of iterations as a four character string with vowels.
You could even test this, by doing something like:
for INDIVIDUAL_ITEM in str(PARAMETER):
if INDIVDUAL_ITEM in "aeiouAEIOU":
PARAMETER = PARAMETER.replace(INDIVIDUAL_ITEM, "")
print "I ran an iteration."
This will print some text to the console each time an iteration has been run, so that you can see how many iterations ran and compare that to the length of the string you gave it.