Most eye-witnesses questioned happen to be at the scene in
question at the time without motive. (Some investigation needs to
be done to ensure that this person is unrelated to any of the
people involved in the crimes.) As such, their testimonies are
usually expected to be truthful because it is unlikely that they
would have any motive for doing so.
Also, as they have personally witnessed these events, a
firsthand account from them would most likely be reliable and
undistorted by a chain of communication as in the instance where a
person claims X told him that his wife said that her mom saw a man
smashing a beer bottle, for example.