I believe its saturated so that you can convert as much alkaloid freebase as possible to fumurates.
I also recall reading that if you add FASI too quick then the fumaric acid precipitates out but if done slowly and steadily it allows it to mix and stays in I have no idea why! Yeah I also recall someone saying FASA isnt as effective as FASI in limo..

The FASI solution itself is saturated (hence the name).

The reason to add dropwise to the solvent is because the slower you add the nicer/bigger/purer crystals are gone.

If you add too fast, it´s like the crystals are `forced` to precipitate fast and therefore all come out as small crystals/powdery, and impurities tend to get trapped inside those crystal formations. When you add it slowly, it tends to former longer purer needles since the slow time allows the precipitating crystal to actually grow on each other.

No real danger of adding too much FASI, excess fumaric acid stays in solution. Even if you are going to change it to freebase later, I still recommend adding FASI slowly because it comes cleaner, and the cleaner the fumarates are, the cleaner the freebase will be.