Both a conventional subwoofer and XBD system will increase the LF extension and headroom of your monitoring system, whilst also reducing distortion, but setting up your subs as an XBD system will also bring further benefits.
When a subwoofer is positioned under a full-range speaker we know the exact position of the two speakers in relation to each other. Setting both speakers to XBD Mode applies the correct crossover and delay settings to perfectly integrate the two speakers together, removing the difficult and time-consuming process that is normally required to correctly integrate a speaker and subwoofer. This means less time worrying about your set-up and more time focusing on your mix.
When an XBD system in linked through the SoundAlign interface, the two speakers unify to appear as a single XBD speaker. This makes it much easier to apply EQs or other settings to both speakers simultaneously.