This expression is used when the speaker wants to state a reason that the listener should also be aware of or when the speaker wants to remind the listener of a reason that he or she has apparently forgotten.
It is used only in conversation and not in written Korean.
Also, it is not used in formal speaking situations.
Details
Kimchi Reader
Indicates that the speaker is reminding the listener of a reason they should be aware or might have forgotten.
Examples
지하철을 탑시다. 출퇴근 시간에는 차가 많이 <span class='focus'>밀리잖아요</span>.
Let's take the subway. There surely are a lot of traffic during rush hours.
그렇게 싸우지 말고 사이좋게 지내. 둘이 <span class='focus'>친구잖아</span>.
Don't fight like that and get along well. You two are friends, you know.
<span class="speaker-a">가:</span> 저 가수를 왜 좋아해요?<br><br><span class="speaker-b">나:</span> 노래도 잘하고 멋있<span class='focus'>잖아요.</span>
가: Why do you like that singer? 나: He sings well and (as you know) is good looking, too.