There are various reasons someone might choose to defend a divorce, three of the most common are:
To delay the divorce if they believe their spouse has given up on the marriage prematurely and might change their mind if given the chance to reconsider or look at options such as a trial separation.
If the divorce petition contains accusations the respondent believes are seriously untrue, e.g. domestic abuse, especially if they believe accepting the allegations might have a bearing on issues such as arrangements for children.
If the respondent wishes to cross-petition i.e. submit their own divorce petition. This might occur, for example, if the original petition states that the marriage broke down due to unreasonable behaviour, but the respondent believes it broken down due to the petitioner’s infidelity.