A legal separation can sometimes cause undue delay for spouses who ultimately want to get a divorce. For example, either spouse can object to a legal separation and proceed with a divorce. That can be a shock for the spouse who put together all of the paperwork for the legal separation.
On the other hand, because Washington is a no-fault state, neither party can object to a divorce if the court has jurisdiction. On that note, a family law attorney with nearly 40 years of experience once said to me that when asked if he wins his cases, he responded, "I have never lost a case . . . you will get divorced."
P.s. Keep in mind that there is a six month waiting period for converting your legal separation to a divorce.