If you clear the stocked product, that means its inventory will not be maintained both physically and financially. It should be cleared ideally for service items. The items with stocked product unchecked are considered as expense product.
Behavior, as per point 1 depends how you are configuring items. Note that service items can have stocked product check in some special cases. When stocked is checked for service items, those items can be include in master planning, though on hand is not maintained.
It is the flexibility we now have regarding to items. You can configure either way which suits requirement.