From the legal POV a judge is duty bound to ensure a Consent Order finalising a financial settlement divorce is "fair" ie complies with s25 Matrimonial Causes Act 1973 and other law.
If there has been full disclosure, both parties took look advice and an agreement is "fair" (see above) a Separation Agreement carries considerable weight. Whilst a separation agreement isn''t legally binding a court will give regard to an agreement as part of the overall circumstances when making an order for financial provision on divorce. A judge can make an enforceable order in the same terms as the agreement.
A Separation Agreement can be useful and save money because the newly separated spouses agree how to separate the finances and divorce a year or two later when emotions aren''t running so high. It reduces the chances of conflict, court proceedings and expensive legal bills.
However, negotiating a Separation Agreement costs about the same as negotiating a Consent Order settling the finances on divorce and you will still need to have a consent order drafted in the same terms as the Separation Agreement. Separation agreements are always subject to review and there is a risk of expensive legal proceedings if later someone changes their mind.