It is particularly useful overseas. I was in Belgium at the weekend and all my € spend was immediately translated into £ so I could clearly see how much I was spending. I could also find the café that I went to for breakfast the previous day because it's location is right there in the Monzo app.
The other useful feature is when I'm out drinking. If I loose the card, I can freeze the card from inside the app. Also it means that the next morning I can see how much I spent.