Nobody likes to feel like their time is not precious or worthwhile.
If a candidate for a position turned up late for an interview and all they said was "Sorry I am late", with no explanation or prior communication as to why they were late this would, quite rightly, not set a very good first impression.
Well, the same applies to the employer/interviewer in terms of that negative first impression.
I have heard a number of times over the years of candidates waiting in the reception areas of firms for long periods after their arranged interview times, only for the interviewer to come through and say something along the lines of "sorry to keep you waiting" - With no explanation as to the reasons as to why they were late for the interview.
This creates the impression of bad time management, being disorganised or the person not really respecting the other persons time.
Even with the best of plans and intentions sometimes delays happen.
Previous meetings overrun, trains and buses are cancelled, accidents happen on the roads leading to delays which may mean you are running a little late.
Slight delays are not usually an issue if you deal with things professionally and communicate the reasons why you were not on time.
The interview process is a 2 way street, employers need to also need to create a positive impression to candidates and it is important to remember that.