What is the point of Counsel's Opinion?

From https://www.legalsecretaryjournal.com/instructing_counsel :

When it is appropriate to use a barrister, the barrister is sent ‘Instructions' (when asked to give an opinion on a case) or a 'Brief' (if the barrister is to appear in court).

Is this so that both the lay client and the solicitor have a written legal opinion (i.e. the Counsel's Opinion) to advise them if they should proceed to trial or not?