I do sometimes have problems with the stem type plants being uprooted by the applesnails because they are so large. They climb on them not to eat them, but to clean the algae off of them.I have mostly soil/gravel substrate tanks. In the tanks with the soil, the plant roots get a little better hold. If your gravel is too large or smooth, the roots don't have much to hang on to, and up they'll come.
As far as what do snails do for planted tanks? Well, the opinions on this vary. Some see them as a pest. I believe they add life to the whole ecosystem of the tank. They clean algae off the sides and off the plants. The eat left over fish food off the bottom and off the plants. I have lots of snails in all my tanks. You can look all over the plants, and in every corner of the tank and see activity going on. When I set up new tanks, the plants and the snails are the first inhabitants. Snails also produce poop just like fish. So they add to the fertilizer that the plants need. You could have a tank with just plants and snails, and everything would grow just fine.
I have of course the large applesnails (Pomecea bridgesii), common pond, tadpole, small ramshorn, and malaysian trumpet snails - these snails burrow down into the substrate and keep it aereated, another benefit.
--Tim