Why do so many men have multiple wives? For example, we have this concept of biblical marriage, but Sarai told him to lay with Hagar. And Jacob married Leah and Rachel. I know it might seem like a silly question, but I am just trying to reconcile it in my head.
This isn't a silly question. In fact it is a serious one. But unfortunately there isn't a nice and tidy answer. So let me give as straight foward an answer as I can.
I believe that the original creation should be treated as paradigmatic (an example and model). This includes one man and one woman relationships (Gen 1:18-25). In a similar way, the first individual with multiple wives is presented in the line of Cain (as oppose to the Godly line of Seth). Lamech is certainly performing some poetic bragging (Gen 4:19-24). But in so doing, he goes well beyond "eye for eye" (Exo 21:23-25). The point of that command was equality in judgment to protect society. Here the line of Cain, via Lamech, is being portrayed as being decisively against both the paradigmatic Adam & Eve while also being a brute to social equality.
So why in the world would we have multiple women and wives throughout the book of Genesis? That's the difficult question right? Perhaps it is unfair but given the creation account, my real question is often "why were these details recorded?". I can imagine that these men sought to ensure a family lineage and thus did not look down on multiple wives. This isn't a shock since the behavior stretches all the way to Paul's day and Paul uses it in his qualifications for elders/deacons (1 Tim 3:1-13). This is not an excuse for their behavior but an acknowledgment that it was socially, though not Biblically, permissible. So as I stated, the question must be "why mention this?".
And my answer is to display in narrative form the destruction it causes. All of Israel's history can be seen of one large example of rebellion against God's purposes and plans. So this too can be seen as a smaller subset of instructional principals in narrative form to confirm God's created order. Lamech was an example of this destructive behavior. Abraham's mistake with Hagar resulted in Ishmael and a large family for him that would forever fight against the promised seed. The incestuous relationship between Lot and his daughters (a serious WTH moment) shows where some of Israel's future enemies would come from. And finally the sons of Rachel and Leah are full of strife their whole lives until Joseph brings them together (a type of Jesus).
But these examples continue outside of the book of Genesis with David and Solomon. Both suffer greatly because of their adulterous behavior and multiple wives. Their sons are merciless and often stupid. Death follows them and destruction ensues. This is only resolved when "David's Son", this time Jesus Himself, marries the church as Adam married Eve (Eph 5:31-32).
So long answer short, I believe the reason these texts are present in our Scriptures is to demonstrate the destructive behavior associated with the practice but also to show that God is capable of working through it all in the final work of Jesus Christ.