Form the article, while the case is being brought by them individually as members, they are alleging the debenture system is oppressive to them as members.
If so, the allegation is similar to when a minority shareholder in a company alleges that the company is treating them oppressively by ignoring them and preferring the interests of the majority shareholders.
On that basis, if the case succeeded it could essentially remove the debenture arrangement and mean that the broad Leagues Club members become able to run for, and vote to choose, the Board. And therefore decide how the Leagues Club is run (including spending on the Wests Tigers etc).