Markers are just a place in the timeline. They provide a quick way to get to a given location.
Arranger sections are a lot more. They define a "container" for all of the track data within their starting and ending location.
Arranger sections are used to modify the arrangement of your song. When you drag a section (using the left side of your screen), it takes all the track data along with it. Same with copying and deleting.
Steven