The Rangers main role is DPS. Assassins may have higher single hits, due to some of their abilities, but the real high ones they have are on a long timer, therefore only used once every few fights.
The Ranger abilities, while being less damaging per hit, are able to be used more often, and will at least equal the assassin per single fight, and then surpassing the Assassin over the course of several fights.
You will also be called on to Start HO's a lot, since most of the scout HO's are the best from a group standpoint. However, thats not just Ranger, thats any of the Scout subclass's.
Coming from the predator line, thats what both the Ranger, and Assassin do... Kill, therefore damage their opponents.