Do you mean how we set the variable before a for loop? like
y = 0
for i in range(x):
y = y+1
why we do y = 0?
if so, its because otherwise when the loop tries to run
y = y+1
it wouldn't have a value for the y on the right side, so it couldn't give an answer.
If that wasn't your question, I'm not sure what you mean—please clarify.