How do I differentiate? Well, do not want to start too artificial classification, but let's try.
The first group: 1 cent flights to Pisa? I've already been to Pisa. Oh, it doesn't matter. The ticket is so cheap that I'll go again.
The second group: 1 cent flights to Pisa? No, I'm not interested in Pisa. I want to go to Paris. Which carrier will offer me the best fares to Paris?