Imagine you have two people, one of whom is a known sociopath and the other of whom is an altruist, and both decide to build their own organic gardening communes.
They each have the same amount of money and a suitable plot of land, and both are able to complete their communes at around the same time. Moreover, let’s stipulate that in addition to the usual building inspections, special attention was paid to insuring that the sociopath’s commune has no hidden torture chambers or anything sordid like that.
Neither person stands to make any significant financial gains from this endeavor; the sociopath, however, has built his commune because the plot of land was desired by a hated rival and for complicated legal reasons the only thing he could get a permit to build was an commune, while the altruist just wanted to provide a place for people who don’t own shoes to raise free-range carrots or something.
Should we care why these two people built their communes? Isn’t the fact that the world now has more quality fruits and vegetables than it did before all that matters ?
In short: no, because motives are epistemically relevant.
It’s not like the altruist or the sociopath built their communes and then killed themselves. Both will go on to act in the future, and knowing why each of them did a certain thing, even when the outcome was the same in every way, will allow us to better predict what their future behavior will be.
It just isn’t feasible to partition off a subset of your motives indefinitely. Ted Bundy might be able to build a handful communes without torture chambers. But if he built a hundred, I’d lay long odds that you’d eventually find a torture chamber on one of them.
And, well, avoiding torture chambers seems like as good a reason as I can think of for trying to discover and track the underlying motives of other people.