From what I know, most EARLY colonies (1400's-1700's) were established primarily for practical reasons. Most exceptions occured when non-great-powers were attempting to make themselves look good, EX: Sweden establishing colonies in the Carribbean. Basically pointless, but allowed them to say, "We've got colonies! w00t!"
It was around the 1850's that people started making landgrabs just for the sake of landgrabbing on a large scale.