Titanium dioxide is normally added to things to make them look white.
Since it is in the capsule, I presume some decided it looked better if the capsule were light in appearance.
Or more likely:
They were concerned about protecting the contents of the capsules from photo-oxidation or other deterioration caused by light. Only a little bit of Titanium dioxide would make the gelatin capsules opaque protecting the contents from light.
Now it happens that I think the light deterioration should not be a problem, since all the D'Adamo supplements are currently packaged in solid white bottles that should not transmit any light.