Most people think of separation as the time period leading up to the final divorce. It can be that, and usually is, but a trial separation period can be used in other situations and for different purposes.
1. You're not really sure what you want. It could be that you and your spouse are having difficulties and have tossed around the idea that maybe you should live separate and apart to think things over. You are worried that this will leave your assets in limbo and that support and custody will not be addressed. It is possible to cover these topics (assets, support and custody) in a large document known as a post-nuptial agreement. A post-nuptial agreement can cover as long or as short a time period as you wish. Should you reconcile with your spouse it can self-destruct. Should you eventually divorce, it can become your final Property Settlement Agreement.
2. You and your spouse live apart by necessity or choice. You may be transferred out of state for employment reasons or, perhaps a terminally ill or disabled relative who lives far away needs your attention. You still consider yourself happily married to your spouse but you need to temporarily relocate for other reasons. A separation may actually strengthen your marriage; for instance, if you work out of state but come home on the weekends, it's as if you are still "dating" and you will find yourself looking forward to reuniting with your spouse.
3. You are certain you are going to divorce and desire to live separately. At some point during a troubled relationship, either one or both parties realizes that things are not going to improve and divorce is inevitable. One party then moves out while the other remains in the marital residence. This time alone will reveal to you what it's like to live alone and be a single parent, and will either confirm or deny that you are making the right decision to divorce.
If you are considering a separation and not sure what to do next, contact an experienced family law attorney in your state.