Excellent question. I view them as a means of changing the reader's focus in order to:
1) Skip over boring bits of the main character's timeline;
or
2) Build tension by creating a cliff hanger situation. (Cliff hangers at the end of chapters build tension only for as long as it takes to turn the page.)
1) Skip over boring bits of the main character's timeline;
or
2) Build tension by creating a cliff hanger situation. (Cliff hangers at the end of chapters build tension only for as long as it takes to turn the page.)