we use กัน when you invite someone to do something. For example, let's have lunch (ไปกินข้าวเที่ยงกัน). But sometimes, there is meaningless. we just say it. I don't know actually why. For example, why don't you guys have lunch? (ทำไมไม่กินข้าวเที่ยงกันล่ะ). You can notice i omit the subject. Also, you can omit the word กัน as well because it will not change the meaning of the sentence.