… if, after using this method, we get different results, it means answer is incorrect ... then we need to make thorough line-by-line check of the working to see where we have made a mistake.

… for questions with very long working, we can even use this method to check part of the working.
for example, if we check at some particular line and get same value as the original expression, we know that at least the front portion of our working is correct !