Riamus explained what it does, but you might be thinking "why do NOTHING with the results?"
The $findfile identifier lets you specify a /command parameter so that mIRC should do something with each file, and it returns the number of files found. The number of files found are often irrelevant as you may just want to list the files or something, in which case you could use /noop to ignore the return value.
This is only one example of where it's useful.
It's also very useful in $regex/$com scripting but I won't go into it as they're pretty complex.
