After decades of empirical studies using the NEO scales, the Eysenck scales, and several others, Costa and McCrae expanded on the Big 5 to include 6 facets within each dimension.  If you were to take the NEO-PI-R, you would receive not 5 scores, but 30.  These 30 scores provide a far more in-depth and valid profile and show that you might be introverted in some ways but extraverted in others. What's even more interesting, Costa and McCrae showed that if you were to take the test repeatedly over a period of years, it's likely that your scores would change in important ways.

With this background, let's look at the six facets of Introversion-Extraversion and see what the benefits might be of having some of these qualities.

Warmth Facet: When you think of introversion, you probably imagine that introverts don't like other people. However, being low on the warmth facet doesn't mean that you don't like other people. Instead, it means that you're hard to get to know and may feel uncomfortable around other people who you don't know well. 

What are introversion's benefits? Although you don't particularly like it when others invade your personal boundaries, you're also not likely to invade the boundaries of others. You show restraint in social situations and may wait to be approached before others approach you.  You may not be the first person someone meets when they go to a party, but you may be the most interesting once someone gets to know you.

Gregariousness Facet: The quality of gregariousness refers to the tendency to enjoy being around others, seek out social situations, and actively try to be where there are plenty of chances to meet new people. People high on this facet of introversion tend to avoid crowds and prefer quiet situations.

What are introversion's benefits? This quality may be framed as a negative in that we often think of the gregarious person as the one most likely to be liked by others. However, when forced to be alone, gregarious people can easily go stir crazy. People low on gregarious instead are just fine being by themselves and involving themselves in quiet contemplation.

Assertiveness Facet: Being assertive means that you are likely to express yourself and see that your needs are met. If the squeaky wheel gets the grease, as the saying goes, then people high on this facet of introversion will wait while others are served.  

What are introversion's benefits? Our society both fosters and rewards assertiveness.  The "assertiveness movement" of the late 1970s turned on a cultural switch that erased politeness and replaced it with brashness. Not getting served fast enough while waiting in line?  Well, then you obviously need to complain, right? Feel like you're being pushed around by your friends? Let them know you don't like it. People high on the assertiveness facet of introversion really don't like to put themselves out there in this manner. They let others lead the way, stay in the background, and keep their opinions to themselves.  Though assertive people often do get reinforced for their behavior, perhaps there are many situations when introverts are much more pleasant to be around.

Activity Level Facet: Your activity level refers, as may seem obvious enough, to how active you like to be. People high on this facet of introversion like to take it easy, are laid back, and react slowly as situations develop. They prefer a leisurely lifestyle to one that involves constantly being on the go.

What are introversion's benefits?  There's one distinct benefit of being high on the introversion end of the activity level facet: You'll be less likely to put your foot in your mouth. Because they react slowly to situations as they develop, they're unlikely to commit the kind of social gaffes that people who have a higher reactance can make. Not only that, but being thoughtful and low key can make you an easier companion than someone who always needs to be on the go.

Excitement Seeking Facet: Psychologists have long known that people vary in the need to be stimulated as well as the desire to take risks. If you're low on the excitement seeking facet, you'll probably never go bungee jumping or become a race car driver.  You seek peace and quiet and are perfectly happy with keeping to your daily routines.

What are introversion's benefits?  Being low in excitement seeking doesn't mean that you are not willing to change or experiment. People low in excitement seeking just don't need to be stimulated by lots of noise and action.  They certainly make better roommates or neighbors because they prefer a steady, quiet, lifestyle.

Positive Emotion Facet: People who are low on this introversion facet find it hard to express joy or happiness, but they are not necessarily anxious, worried, or depressed.  They can feel inwardly contented without needing to share their pleasure in their outward behavior.

What are introversion's benefits?  It's possible to enjoy life quietly without feeling the need to emote outwardly your positive feelings.  Unless you're also high on the Neuroticism facets of involving worry and depression, you can still be happy. You just don't feel the need or desire to express your happiness.

Summing it up:

The majority of people may score high on one or two Extraversion-Introversion facets, but rarely do they have high scores on all six. Unfortunately, most of what you read about in the popular press portrays the negative qualities of introversion. Some people incorrectly compare introversion  to social anxiety disorder (social phobia). They are not the same.  Introversion can involve extreme shyness but not necessarily unhappiness. Moreover, introversion interacts with other dimensions from the Big 5, such as neuroticism. You can't look at one of the Big 5 dimensions without looking at the other four.