I felt it was easier to read and understand, since the "is-" or "has-" clearly shows it's a modifier class, and not a base class.

Also, it prevents the namespace from being "polluted" by class names that could be used for other things. For example, there's a "box" element, and a "is-boxed" modifier for the "navbar".

Lastly, the same modifier class can be used on different elements, since they are specific to the context in which they're used.