Behavior filters have no order dependency, which means that the "user" can put them on the element in any order, for example:
<form data-behavior="FormRequest FormValidator">
In this example, the form request instance will be set up and then the validator instance. But it could easily have been the other way around. As such, the filters can't be written to expect any sort of order of operations. This is where Filter Plugins come into play. Plugins DO get an order of operation promise. That is, the filter they modify is ALWAYS applied before they are. So when you need two things to work together, the plugins are the glue.