Some states require that a couple seeking a divorce have been separated for a period of time, so a legal separation would be needed.
Some couples need to be separated, but they need to remain legally married, perhaps so one can continue to be insured for medical other or purposes and a formal legal separation makes this possible.
At times there is no question that a couple is moving toward a divorce, but they know it will take some time to work everything out. If their incomes are substantially different, it may be worth it to develop a written separation agreement so the person paying alimony can deduct the alimony on their tax return. The paying spouse might be able to pay the receiving spouse more than enough to pay the tax on the alimony and still come out ahead.
If one of the spouses has a religious objection to divorce a formal legal separation may allow the spouses to remain married even as they live apart.
Beyond that, there may not be much of a reason to go through the time, pain and expense of a formal legal separation. It may be better to agree to reach a working arrangement for an informal separation and then follow it up directly with either reconciliation or divorce.