Why is it important for an accused person to be able to see the witnesses against him in court?
It is a constitutional right to confront one's accuser(s). The
intent is to prevent anonymous and fraudulent accusations which the
defendant cannot challenge. This leads to the "hearsay" rule.
Hearsay is rarely allowed in court. I say "rarely" because there
are some exceptions (at least, in Texas). The problem with hearsay
is that it cannot be challenged, thus violating the right to
confront one's accuser.