When things don't work out
Some parties do not want or need to completely end their marriage, but need to separate legally and physically. In this case, the parties can get a judgment of Legal Separation rather than a divorce. This is different than being “legally separated” for the purpose of calculating pension entitlements or responsibility for bills.
To be “legally separated” and establish an official “Date of Separation,” the parties have to act in a way that observers would realize the marriage was ending. This issue sometimes makes enough of a difference that it will require a court to decide, but most cases will not have a serious dispute, nor would it make a difference.
The two common reasons to seek a Judgment of Legal Separation rather than a divorce are to maintain entitlement to benefits like health insurance or a personal distaste for divorce. There are several procedural differences between a Legal Separation and a Dissolution. A Dissolution has strict residence requirements, while a Petition for Legal Separation can be filed immediately after moving into California (and then modified to a Dissolution after the residence requirements are met).
The Judgment of Legal Separation has no time requirement, while a Judgment of Dissolution makes the parties single only after six months from service of the Petition. Finally, after the judgment is entered, the parties are still married and cannot legally remarry. To become single, they must file a new Petition to address that issue.