Most edit boxes pass the string you enter into some sort of processing or database. Within the processing process or the database table, the code expects some sane limit on the amount of text entered.
Now “sane” is open to discussion, but I’m guessing you’re probably not planning for this many:
William Shakespeare’s Hamlet, stripped of line breaks and punctuation, contains 135,014 of the most thought-provoking characters in the English language. This should exceed the limits of most individual controls unless you’re testing a word processor.
I ought to try that on some of our 40-character fields, just to watch the database stand and unfold itself.