Lying occurs frequently in this novel. Curiously, some lies, like those Huck tells to save Jim, seem to be bad. What is the difference? are both wrong?
Why does so much lying go on in Huckleberry Finn?
Some of the lying happens because Huck is the average American boy. Kids like to create adventures, and lying is certainly an excellent way to do that.
As far as Huck's useful lies - most of them depend on which side of the moral fence you sit on. If you believe that Huck is doing the right thing in running away from his father and/or in helping Jim become free, then most of the lies he tells are necessary. Those lies can be traced all the way back to Huck pretending to be a little girl in order to find out if they've been discovered on Jackson's Island.
However, if you believe that Huck should have been obeying the law of the time and should have stayed with Pap and returned Jim, then his lies were just for fun.
As far as the enormity of the lies Huck creates, I believe that is all part of the characterization of Huck. Twain creates a character so over the top that a reader can't help but admire his imagination. This novel certainly wouldn't be as entertaining after 150 years if Huck told sensible, credible stories. Some scenes, like his lies about life in England (which he swears to on the dictionary), simply portray Huck as an extremely creative kid - one of the great things about reading this novel!