God created humanity with free will so that we may come to love Him freely and in truth.
However, since we have that free will, we can sin against Him.
God knew this would happen when He created us, so He provided a way for us to be reconciled with Him for free.
Hid did this by incarnating as a man to pay for our crimes against Him, Himself.

Why was this necessary?

It was necessary because God is perfectly Good and Just.
Since He is perfectly Good and Just, then there must be retribution for the crimes we have committed.
His Justice must be satisfied.

But since God loves us and wants to have a relationship with us, He took it upon Himself to suffer in our place for our sins.
This is the truest expression of His mercy for us.
All we need to do is accept this free Gift.

Some might ask,"If Jesus bore the sins of all mankind, then why is there as hell?"

If you do not believe in Christ and allow Him to cover your sins, when you stand before God, you will have to pay for them yourself.