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.