[merijnv]

Because scientists are aggressively selective in what they read (you have to be, given the volume of text produced) and skip large parts of the paper to see if they're interesting. A very common paper reading approach is:

  1. Read abstract. Interesting? Go to 2, else stop reading.
  2. Read introduction. Still interesting? Go to 3, else stop reading.
  3. Read conclusion. Still interesting? Read the rest of the paper, else stop reading.

The conclusion is there to summarise everything and let people decide whether they want to bother reading the entire thing.