Somebody explain poligamy to me

Why does the poligamy exist? Whats the point of having poligamy?<br><br>For example what does it happen if you take a man who is married to a woman and then he falls in love with another girl and he wants to marry her. He can marry her but comitts crime? How does this happen? How it works?<br><br>Every answer is good for me. I just wanna understand what is going on.