Guy Glantser explains what schema binding is and why it’s important:
In SQL Server, when we use the “WITH SCHEMABINDING” clause in the definition of an object (view or function), we bind the object to the schema of all the underlying tables and views. This means that the underlying tables and views cannot be modified in a way that would affect the definition of the schema-bound object. It also means that the underlying objects cannot be dropped. We can still modify those tables or views, as long as we don’t affect the definition of the schema-bound object (view or function).
At one point, I was a lot more gung-ho about schema binding and would do it even for views that didn’t require it, as I liked the idea of protecting the schema from changing. This turned out to be more of an annoyance than a benefit, though, so I quietly modified that belief a while ago. Nowadays, I’ll typically use schema binding when it’s required and on the rare occasion in which I create a function (assuming the function complies).