Why does clickbait use numbers? Around the Internet, especially in social media, we see examples of hyperlinks in a common format that has been cynically dubbed "click bait" or "link bait". 

The idea is that authors publish these links with evocative titles, trying to entice a casual browser into clicking the link and viewing the content (where the resultant loading of the advertisements presumably earn some small amount of money for the content provider).

Some examples of click bait (with links removed, to protect everyone's sanity) are:


  
  Watch out, these 15 baby penguins are waddling off to steal your heart. 
  31 creative life hacks every girl should know
  20 Powerful D-Day Photos On The 70th Anniversary Of The Invasion
  11 Sexy Star Wars Tattoos
  


Many of these kind of links use provocative phrasing, like "you won't believe" and "will shock you". I can see intuitively why this works to entice people to click the link.

What is the reason for giving a number, like in the examples? Is there any psychology involved in this trend?



This question comes from someone who doesn't really know anything about psychology or cognitive science; I'm just interested.
Digital Marketing often produces 'content' for users to consume. 
That content is ment to catch and keep the attention of the user, in an attempt to make the user trust and follow the sender (usually a website or blog). 
When the user shows engagement like clicking the right buttons, the user will somehow be tempted with some kind of offer (a newsletter, a free ebook, etc). If the user takes that bait, there will later be an offer to actually buy something.

The whole idea here is to catch and keep the users' attention.
One of the most common ways to achieve this, is by making lists of something the user might want to read\see. 
In order to have users click(open) the content to begin with, the writer needs to make the content seem valuable in only 1 short line.
By typing the number of entries on that list, the writer is trying to make a point: There's stuff to see her, come check it out!