This is just a curiosity question. I was just listening to a clip where a pilot did a go-around at KSFO. The controllers queried the pilot at least 5 times for the reason for the go-around and I have heard this in other clips also. This pilot finally answered that his approach was too high, fast and unstable. Exactly why does a controller ask this question? The pilot felt it was necessary for the safety of the flight would seem to be the answer in most any case.
One reason they ask is because depending on the circumstances, they may need to warn the aircrafts on final behind them of the issue. For example: Let's say that that pilot in question performed the go-around due to a flock of birds on short final, or even at the at the REILs. If the pilot just felt it was necessary and that was his answer, the arrivals behind him may end up in that flock of birds, leading to a blown engine.
Another reason: weather. Wind shear, microburst, severe gains or losses of airspeed on final.. all of those are invisible barriers that aircraft on final nor ATC would be able to see that can cause a pilot to go around. And those are worth giving the caution to other aircraft on final so they can be ready for it.
It's been said before: While pilots are PIC of their aircraft, ATC is in control of the airspace that they are flying in, and need to keep it safe with aircraft properly separated to ensure the safety of the airspace they control. Knowing what happens on a go-around is key to ensuring that safety is upheld.