In a benefit performance the performers (and, likely, the crew, director, holder of copyright, etc.) usually do not earn any money.
Instead, the takings will go to raise money for (to "benefit") some mutually agreed upon cause: e.g. the Actors Benevolent Fund; a hospital; a foundation. In other cases, a benefit performance is given to help the writer or a performer out of financial difficulties. In the 19th century, theatre companies routinely held benefit performances for longstanding members of the theatre's troupe. Benefits were also sometimes given for retiring actors.