Understanding our type's functional preferences gives us specific insight into our strengths, weaknesses, and preferences.
The functional leanings of a type help us understand that we are mentally predisposed to be good at certain things and to be bad at certain things. A type who has Extraverted Thinking (i.e. ESTj, ENTj) as base function (see Model A) will have a large capacity to be productive and should feel confident in taking a leadership role in technical areas. These types also have Extraverted Feeling as a role function, meaning that while they are Extraverted types and will be comfortable in leadership roles, their capacity for sustained, boisterous group socialization isn't that great. Instead, they'll gravitate towards their suggestive function (Introverted Feeling), meaning that they'll be more inclined towards closer, more personal social contact.
This last point is actually quite helpful for Introverts in understanding their preferred mode of socialization. For example, an INTj (technically-oriented, structured personality) has Extraverted Feeling as his/her suggestive function and should thus seek out warm, even raucous social situations (or people) to be able to open up socially (whereas a more direct and personal approach might be a bit awkward).
Understanding personality theory helps us deal with people.
And I don't necessarily mean that in a negative way (i.e. how to I deal with this person that I hate...). Rather, knowing someone's type makes it easier for us to give them information, help them out, or process the information they are giving us. Knowing someone's base and creative types means that we can present information to them in a way that they'll easily understand. Someone with Introverted Logic as a base type (that INTj, for example, or an ISTj) will be very comfortable with a detailed breakdown of a situation or problem's facts that they can consider, whereas someone with Extraverted Logic as a base type (ESTj and ENTj) will be content with just enough information to understand a situation at a high-level, followed by a discussion of how to resolve it as efficiently as possible, no time wasted. Or, from a completely different perspective, someone with Introverted Feeling as a base type (INFj, ISFj) will be more concerned with how a situation affects the people in it, emotionally or ethically. Similarly, knowing a person's role and suggestive types makes it easier to recognize where they might need help--those ESTj's and ENTj's we've been talking about, who have Introverted Feeling as their base type--they'll be quite relieved and satisfied when you take the initiative in creating closeness/intimacy with them, as long as you are (or can feign) sincerity.
...I'm skirting the line that turns this into a "how to manipulate people," article, so I'll end things here, for now.