The importance of sparring

Sparring teaches "open skills" that require quick decision making in a static environment. Technique drills, what some martial arts call kata, teach "closed skills", which means you know just what will happen, what you must do, and when.

Open skill practice may improve the brain in important ways that closed skill practice does not.

One article mentions a study on how older fencers (who would have sparred a lot), had perceptual and reaction abilities that did not degrade with age like people who did not practice an open skill. Another found a similar difference between tennis (open) and swimming (closed). Excellent reading.

This is your brain on fencing: How certain sports may aid the aging brain 

Science may be able to explain what’s going on in Michael’s aging brain when he’s on the fencing strip. A small but growing body of research suggests that fencing and other sports that require quick decision-making may improve cognition in both young and old people, and help stave off certain mental declines associated with aging.
In addition to fencing, sports that use what are called “open motor skills” include basketball, hockey, football and table tennis. “Closed motor skills” are those used in a stable, predictable setting in which the performer typically chooses when to start using the skill and knows exactly what to do. Closed-skill sports, which include biking, bowling, golf and gymnastics, involve self-paced movements.

http://www.washingtonpost.com/national/health-science/this-is-your-brain-on-fencing-how-certain-sports-may-aid-the-aging-brain/2015/04/06/92b70970-c98c-11e4-b2a1-bed1aaea2816_story.html?postshare=81428430695138

Neural Correlates of Attentional and Executive Processing in Middle-Age Fencers

http://journals.lww.com/acsm-msse/Fulltext/2012/06000/Neural_Correlates_of_Attentional_and_Executive.11.aspx

Open vs. Closed Skill Sports and the Modulation of Inhibitory Control

Here we compared inhibitory control across tennis players, swimmers and sedentary non-athletic controls using a stop-signal task without a sport-specific design. Our primary finding showed that tennis players had shorter stop-signal reaction times (SSRTs) when compared to swimmers and sedentary controls, whereas no difference was found between swimmers and sedentary controls.  

 In general, sports may be categorized into two types: open skill and closed skill sports. Open skill sports are defined as those in which players are required to react in a dynamically changing, unpredictable and externally-paced environment (e.g., basketball, tennis, fencing and etc.) [20]. By contrast, closed skill sports are defined as those in which the sporting environment it is relatively highly consistent, predictable, and self-paced for players (e.g., running, swimming) [4], [20]. Athletes from open skill sports may develop more flexibility in visual attention, decision making and action execution [21], relative to athletes from closed skill sports. 

(emphasis mine AH)

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0055773