The reason that St. Clair always disguised himself was that he could make a lot of money by doing so.
St. Clair was a good actor who found out that he was able to impersonate a disfigured beggar with a twisted face. By doing so, he was able to make a lot of money. This allowed him to marry and to buy a very nice home in the country. He would not have been able to live so well on the salary of a journalist.
So, by disguising himself as a disfigured man, he was able to make much more money than he could any other way.