It has diverse diagram elements (classes, interfaces, etc.) already preconfigured for quicker insertion and simpler editing.
These elements can be classified by priority, purpose, or by predefined type, so that the visual isn't the only way to control and access the properties of each one.
Thus, each one of them allows, beyond the configuration of different flags and properties, the an annexation of notes and even open-source tools that without a doubt will be very helpful when the time comes for implementation.
Thus, ArgoUML is a utility that, being open source, has lots of reasons to boast among other similar paid tools.