I don't think "plan an explosion" is quite right.

They planned a test that would subject various components to stress levels outside of the normal mission profile. The various specific failures that resulted from that may be within expectations but not necessarily planned.

In engineering you want to know that a design will not just succeed at its rated limit, but have some margin percentage of safety above that. To measure that margin often involves destructive tests.

SpaceX's development methods differ a bit from more traditional rocket development by performing some of these potentially destructive tests with full-scale articles in real flight scenarios as part of an iterative process.

I'd say it's a mixture of chaos engineering and load testing. They know what performance they need for a safe flight, but they want to know how much margin of error they've got (and how close their simulations are), and what they can do to optimise.

It's a different apporach to say the Apollo program, where they did heavy up-front analysis, at the expense of cost-efficiency, speed, and innovation. They had one-shot for a flight, otherwise that's several $bn up in flames.

Even with the last few mishaps, it's an approach that seems to be working. If you look at Starship and Falcon's journey in comparison to SLS and Blue Origin, they have done so much in such a short timespan.

It's very common for extremely hot metal pressure tanks to rupture when plunged into water.