Whatever they say about their reasons, this was effectively a test of a weapon system which could wipe out continents in a single strike.
Find a big thing which grazes Earth (such as Apophis) and smash a small thing into in a very carefully calibrated manner. Now you have a weapon to take out thousands of square miles at a location of your choosing. And there’s many objects out there larger than Apophis, some of them large enough to melt continents.
This experiment provides data for the calibrating of future deflections.
That’s very dicey stuff. There are a lot of uncertainties about the composition of these bodies, from one to another, essentially boiling down to how solid, cohesive, and homogeneous they are. At the very high speeds and energy densities involved in a collision, a rather extreme form of fluid dynamics is in play. We DO understand quite a bit about those dynamics, given a known target material, but, if the material varies from asteroid to asteroid, and quite possibly laterally and with depth within an asteroid, unless a target point on the proposed “weapon” asteroid was carefully scrutinized (and at some depth), with some additional analysis of the rest of the body, smacking the asteroid into Earth within a few hundred miles of a given spot would be highly unlikely.
All this activity, plus the shove of the asteroid itself would have to somehow be kept secret / unobserved.
The uncertainties about the target body are part of why the results in this particular test had such a very wide range of possibilities. That can be narrowed down a bit, but overall the whole idea is pretty impractical with technology humans will have in any less than 100 years, at the rate we are going presently.