Why was I asked for an interview? Since interviewing
takes time and costs money, it is only done for candidates who look so
good on paper that the persons doing the hiring want to make sure that
the person in the flesh resembles the one described in the vita and
letters of recommendation. In the case of postdoc job, often the best
candidate is brought in; if the candidate checks out well on the
interview, an offer will be made. If not, they will go on to the
second candidate. In the case of a faculty or industrial position, a
few candidates will be brought in; the one making the best impression
will usually be hired.
Is there any reason not to accept all interview trips? If
you already have offers from places you prefer over the one now
inviting you, decline the trip honestly and graciously. If information
you have learned since you applied strongly suggests that this place is
inappropriate for you, decline honestly and graciously. The reason for
the honesty is that this gives the place a chance to say: we think we
are better or more appropriate than you believe; we want to prove this
on your visit.