Rutherford got involved with the IBSA out of personal interest and he became deeper involved with Russell when he choose him to do some legal work representing him and the WTS.
One should keep in mind that the IBSA then was making bold proclamations that Christ has returned plastered on the front page of the Watchtower.
When he eventually took control over the WTS he made himself into a crooked self serving religious charlatan and used the money for himself that Russell had left in the WTS.
I guess you could say he could see the advantage and opportunity for himself, the power, control and money was all there.