In the PHB aside from the proficiencies, languages, and equipment the only thing it gives you is researcher. Do you get any benefits from the specialty you take? For example, if you take or roll the alchemist specialty but don't get a proficiency with alchemist tools it doesn't seem to really line up a good back story which defeats the purpose.
The Specialty is purely flavor, and doesn't provide any mechanical benefit. The knowledge that a Sage provides isn't meant to be practical knowledge, but more scholarly.
For example, a professor of anatomy might know the name of every single bone in your body, but that doesn't necessarily mean that he'd be any help setting a broken bone. In the same way, you might be a Sage specializing in Alchemy, but that doesn't mean that you've spent years mixing and testing chemicals. It means that you've spent years reading theoretical papers and reports from actual alchemists.
If someone wants to find the recipe for transmuting lead into gold, you might be able to help them or at least point them in the right direction, but if someone comes to you with the esoteric materials required to actually pull that off, you wouldn't have the expertise needed to mix the chemicals with the precision necessary to not blow yourself up or fill the room with noxious fumes.
One of the big advantages of Sage, is if you take a fairly broad field of speciality, you can ask the DM almost any question and receive an answer or know where to find the answer to that question.
Yes you can't practically apply the knowledge you have of a specific skill such as Alchemy but you know far more lore than most Alchemists and are more likely to know the result of mixing certain ingredients in which quantities will give the best results. Whereas an alchemist would often be a case of trial and error.
And as they say, "Knowing is half the battle."