I want to create a unified design for the slides used in my organisation. When do you recommend the use of either a master slide or a slide library?
Most likely you'll want both. All PowerPoint presentations have one or more master slides, so the starting point is to make the master for your organisation look the way you want it to. You can also save the master as a template that includes pre-made slides. When a user starts a new presentation based on your template, they'll get not only the slide master(s) but any included slides, which they can then edit as needed.
If there are large numbers of pre-made slides, only some of which will be useful to any given user for a particular presentation, a slide library would come into play. There's quite a variety of software that provides this kind of function in various ways.
There are also add-ins that can automatically add chosen slides for a user, apply your organisation's branding and so on ... while it's not possible truly to force a PPT user to do what you want, an add-in can make it simple for a user to follow branding standards; so simple that they've no excuse for NOT following them.