I know the basic advantage to blowguns is that you can't accidentally poison yourself when applying poison to the darts, but is that really it? I feel like delivering the poison via d8 crossbow with a 5% chance to poison yourself would be better than a d2 blowgun with a 0% chance of poisoning yourself. Unless the flavor text "They are nearly silent when fired" is implying that it would be harder for your target or anyone who didn't see the dart to locate you. Oh I should also mention CRB only.
A creature needs only the feat Simple Weapon Proficiency and a blowgun to make all of its iterative ranged attacks
The blowgun is the only simple ranged weapon capable of making iterative attacks without devoting additional resources to doing so. Unlike a crossbow or sling, no extra time's needed to load a blowgun. Unlike a thrown weapon, the feat Quick Draw isn't needed to retrieve a replacement as a free action after each attack.
This is actually a significant change for Pathfinder as Dungeons and Dragons 3.5 had no such weapon in the Player's Handbook, leaving the DM's intelligent but feat-starved monsters' iterative ranged attacks much more difficult to use effectively.
Slightly off because this is not a direct answer, but if I may, let me examine the premise of this question a bit...
There does not always need to be "a point" to a weapon, in the sense that every weapon is superior to all other weapons in some game technical fashion. Weapons have been invented in different cultures and at different points in time. Because of this, chances are overwhelming that some weapons are inferior to other weapons in every way that a game system might hope to simulate.
However, this does not make a weapon useless or less cool. E.g. those tree-dwelling, monkey-like assassins will surely give a different impression if they are wielding sleek blowpipes instead of clunky crossbows.
I guess my take on this is that "the point" of a weapon does not need to come from statistics alone.