Not only does it tell you why it chooses a number but it tells you the reasons attending to 14 different simple and advanced Sudoku patterns. Everything from simple forces and pins up to mega-expert techniques like forcing loops and chains.
It is a good helper because It can give you hints on how to proceed, or solve the puzzle by human-style logic, with detailed explanations of the steps. No known puzzle can stump the Susser's heuristic deduction engine. When you have used it some times you'll be able to solve it on your own and go step by step to solve more difficult puzzles.
Finally, you can download sudoku puzzles for free or you can drag Sudoku graphics from just about any webpage and they'll be scanned and loaded into the application.