Do your fingers type main when you mean mien or mane?
Has your hero walked into the Capital Building instead of the Capitol Building? Or pulled out his wallet to check his capitol instead of his capital?
Does your character ask people if their coming to his party? Perhaps she points to the villain who's standing right over they're?
These are just a few of the things you may catch on a back-to-front read. Since this changes your perspective, you are more likely to see what's really on the page instead of what you know you typed there. Sometimes our eyes are so good at completing patterns that even reading from the end to the beginning isn't enough--we may need to read each sentence backwards to catch some of those pesky typos.
Be sure to also print your book, then do a read-through from beginning to end, preferably aloud, to hear the flow and make sure that what you said sounds like what you meant to convey. Sometimes even the right word sounds wrong when read aloud.