The heart of this build is the Assasin Rogue’s ability to deal an Advantage attack to any enemies that haven’t taken a turn yet. On its own, that ability makes it fairly easy for the Assassin Rogue to enter Stealth, get into Ranged attack…err…range, and then unleash a devastating opening shot upon an unprepared enemy.
However, by multiclassing into the Gloom Stalker Ranger, we eventually not only gain additional initial attack bonuses and a valuable “Hide” ability but the ability to perform a free extra attack at the start of combat.
When you combine all of those attributes, you not only have a high chance of dealing a devastating opening blow but dealing multiple devastating opening blows that can kill several enemies before the fight even properly starts. Even when you can’t reach the conditions needed to gain the maximum advantage in such situations, this particular Rogue build offers high-end ranged damage and all of the other benefits you get from the Rogue class (free actions, stealth, etc.)
While you don’t really start to see the full power of this build until Level 8 (when you unlock both subclasses) and Level 10 (when you gain the additional attack benefits), it really is quite useful throughout. At worst, you’re going to be a very stealthy archer who is also capable of utilizing some of the Rogue class’ more beneficial role-playing/out-of-combat attributes.
It should be noted that some variations of this build actually pick up Fighter class abilities for those Level 11 and 12 slots. While doing so will eventually grant you another extra attack ability, that ability can only be used once per day. I think it’s best to pick up that extra Rogue spots at the end of the leveling process in order to acquire the class’ Evasion ability (an incredibly defensive perk), though your mileage may vary.
Finally, you can swap these classes a bit and go Ranger at the start and pick up Rogue later if you’d like. Again, I find that the Rogue’s class benefits and early bonuses make it slightly more valuable to use them as your base class, but the fact that you acquire a Rogue as one of your first companions means that you can easily justify starting as a Ranger. Ultimately, though, it might come down to personal preferences.