Yes, the pass is much cheaper than buying all the tickets you may need individually, and in many cases, a pass that allows unlimited travel on any of the 3 days at 30 is cheaper than a single ticket you intend to buy . Especially if you use the free pass for children. (You can also get a pass for a British escort if you visit someone and then travel with them).
Also, with the pass you do not need to book a specific time and worry about losing your train. And the premium for the 1st class is less than the premium on individual tickets.
The only drawbacks (I have used these passes, mostly London Plus, for 5 years or more once or twice a year) are that you can not go through the automatic ticket barriers, but you have to find one where you can see the Pass ( or, in my experience, the cover / envelope in which it comes) to an assistant, and can not reserve a specific seat. I receive first class passes and have never had problems finding a seat not reserved for sitting. You also have to ask for it while you are still at home and have it sent to your non-British address, so you should plan ahead. . And you'll have to line up at the first train station you arrive at to validate it. These are very minor inconveniences.