There are two important use cases for this arrangement:

(1) To test the Einstein Toolkit; by re-caculating a knows solution one confirms that the spacetime geometry solver in the Einstein Toolkit is working as expected.

(2) To analyse these spacetimes or to evolve test fields in them. For example, we can study the behaviour of horizons in known solutions, or trace geodesics, or evolve test matter fields in these spacetimes. Those calculations might otherwise be impossible, even though the geometry of the spacetime is known analytically. See e.g. <https://arxiv.org/abs/gr-qc/0511017> or <https://arxiv.org/abs/1007.2990> for examples.

-erik