It means not only does the sealant drain from the valve back into the tyre, but it also avoids it being sucked from the pool at the bottom of the tyre into the valve when air is released. At least that's why I do it that way.
The 60ml or so of sealant at the bottom of the tyre is nothing like enough for the valve stem to touch it. I reckon I'd need to put about 500ml in there for that to happen with an inflated 38mm tyre.
Even with a 25mm tyre, I think it's unlikely with normal amounts of sealant.
It means not only does the sealant drain from the valve back into the tyre, but it also avoids it being sucked from the pool at the bottom of the tyre into the valve when air is released. At least that's why I do it that way.