It is says that S.W. Erdnase hide his real name by spelling it backwards S.W Erdnase backwards is spelled
E.S Andrews
Why did he need to hide his name
I think any attempt to answer this question would just be speculation, but we can look at potential reasons why someone, in general, may wish to use a nom de plume, and then see how they fit what we know of Erdnase.
To my mind, it seems possible that the author (who according to Marshall D Smith, was in his early forties in 1901), had decided to settle down, perhaps with a wife, and leave his former illicit career behind. At this "mid-life crisis" age, he maybe wanted to leave something tangible to the world, rather than just a string of impecunious suckers. As a card cheat, he was used to hiding his great skill in manipulation, and it would make perfect sense to conceal his true identity, in case he ever wanted to return to his former profession. As he says himself, "Excessive vanity proves the undoing of many experts", and he held true to this maxim, even when writing his magnum opus. Perhaps he intended to reveal his identity later in his life, but was unable to for some reason.
Anyway, as I say, just speculation, but it's an interesting thought-experiment to come up with these sort of ideas then test them against viable Erdnase candidates. As long as you don't fall into the trap of "twisting facts to suit your theories", as Sherlock Holmes put it.