Higher precision-The flat but tapered brush heads facilitates smoothing and blending over both larger surface plains (cheeks forehead) as well as smaller more awkward facial zones (eye socket's inner corner of eye and bridge of nose)

Ease- Although experienced fingers or a wedge shaped sponge can do the trick, I have personally found, that a foundation brush lets you access the face with a lot more ease.  The paint brush styling of the brush is a familiar action for most people and with a little blending practice, easy to master.  
TIP:  Use a Q-tip to apply the foundation, then blend out with a brush changing the stroke angles for more effective blending.

Quick-The dense soft but firm bristle can blend larger amounts of foundation.  With liquid foundation you do have a more pressing time line because of how fast it can dry. 
TIP:  As a beginner you will want to have an application strategy.  Dot your foundation with a  Q-tip and blend, one zone at a time, till you get a handle on how much coverage you want; the  amount of foundation you want dispensed in each zone and the blending required for it, to achieve a sheer undetectable finish.