The intercessors, yeah they have extra toughness and a extra wound, but just fall flat with no special weapons and no transports. They are immobile and while they would be great if they could get in land raiders/stormwolfs to hold objectives, being unable to restricts them greatly.
Reivers offer a support role with being able to get into position quickly and holding their own in a fight. They also have a range weapon making them more versatile than the intercessors.
Pretty sure you meant that Reivers have a melee weapon that makes them more versatile.