4. Data Binding

Any application has data that it's allowing the user to manipulate, whether it's from object, hierarchical or relational sources. Regardless of where it comes from or the format it's in, there are several things that you'll most likely need to do with the data, including showing it, converting it, sorting it, filtering it, relating one part of it to another part and, more often then not, editing it. Without some kind of data-binding engine, you're going to be writing a great deal of code to manually shuttle the data back and forth between the data and the UI. With data binding in place, you get more features with less code, which is always a nice place to be.