There are a lot of similarities between a legal separation and a divorce. On a practical level, there are few differences - you still have to negotiate child custody and support, as well as some other things. The differences is in the legalities. In a separation you retain the legal benefits of marriage, even though you and your spouse are living separately.
Often people take this option when they think they might reconcile after a time, and the separation gives both parties some space. Some might also choose to separate if they feel they cannot live with the person, but strongly object to divorce on moral or religious grounds.
There could also be financial reasons where it benefits on or both parties to be separated and retain the rights of a married person. If one or both are near to being eligible for government benefits like social security, they will get a higher amount if married than if not, so a separation would be better. There are also tax benefits to being married, so people might choose a legal separation.
One benefit of a separation is that it protects both spouses and their property, and protects both from debts the other person might take. While married you can be held liable for our spouses debts, while you cannot if separated. However, some states take into consideration the intent of both parties. If the two do not intend to get a divorce, but are separated for other reasons, they can be held liable for each other's debts.
Each state has its own interpretations of when property is joint or separate, or whether the intent matters. It is good to check the laws of your individual state on these matters. But in general, a separation allows you to retain benefits of marriage, and some protection of your property until the divorce is final.
Of course, you may also do a temporary separation with no court involvement. Some states do not recognize legal separation.