Here is what beersmith gives me about Malto Dextrin:
Malt bases sugar that is less than 5% fermentable. Increases the body and mouth-feel of the beer and adds slight sweetness.
When I put it in a recipe, it does not change any off the data, i.e. SRM, Gravity, etc...
Like the description says, it is mainly used for mouth-feel and to add some sweetness.