I am invisible, understand, simply because people refuse to see me.
--Ralph Ellison
Different reasons:
On Reach, they needed a ground invasion to take out the generators for the orbital MACs and then to raid Castle Base to find the Forerunner artifact.
On Harvest, they were looking for Forerunner artifacts.
On Earth, they were finding the portal to The Ark and apparently were looking for other things (in Ghosts of Onyx Blue Team was doing missions to stop the Covenant in various places around Earth).
On Sigma Octanus IV (spelling may be off there), they were looking for the rock in the museum so that they could locate the Halo rings.
On Onyx, they were trying to find the portal to the Shield World.
It seemingly always had a purpose.