It's rude to show up unexpected.
Cindy is at home bored and wants something to do.
Cindy wants to visit her friend Jimmy.
Cindy decides to send Jimmy an email to ask if she can come over.
Cindy walks all the way to Jimmy's house and knocks at the door.
Jimmy replies that he was about to go to the library, but he'd much rather spend time with Cindy.
Turns out Jimmy's not home and Cindy walked all that way for nothing.