Throughout If on a winter's night a traveler, we get to read the first chapters of ten different novels. Frustrating, yes, but there's a point. Did you notice that all of the protagonists are kind of similar?
For starters, they're all male and usually pretty young. Plus, they all focus their attention on women as sexual objects—with increasing pushiness as you come to the later novels like Without fear of wind or vertigo or On a carpet of leaves illuminated by the moon. The focus on women is directly connected to you, the Reader, and your desperate need to know what happens next in the stories you read. Remember, whether it's women or stories, you (as the main character of Calvino's book) are always after something that keeps getting away from you.
We're guessing, though, that the real reason these characters exist is to get you engrossed in what's happening to them, just so Calvino can then interrupt their stories at pivotal moments. By doing this, Calvino strips you of your normal expectations for reading and bashes you over the head with frustration ten times in a row. It seems like he's hoping to shift your focus onto the potential of reading so you can find even more pleasure in it.
In order of appearance, here are the ten protagonists:
(1) Unnamed Man
(4) Alex Zinnober
(5) Ruedi the Swiss
(6) Unnamed Professor
(7) Unnamed Wealthy Man
(8) Unnamed Student of Mr. Okeda
(10) Unnamed Man