None. It has the benefit of being quick and trouble free. Remember that it can be pleaded by a petitioning wife as ' adultery with a woman unknown' and, 'with a man unknown' by a husband. This plea may be cheerfully accepted with an admission of adultery by the respondent. The 'third party' remains anonymous. The petitioner is well advised to plead that way without naming anyone, even when that person is known. Indeed, the cynic may say that no adultery needs to have occurred; I am not advocating lying, but I am sure some people prefer that to lengthy allegations about their unreasonable behaviour !
The court is not really concerned about the rights and wrongs in such cases but only needs to know that the marriage has irretrievably broken down. The real concern is about the welfare of children first and then what financial matters arise over division of matrimonial property and any claim for maintenance