Will this be on The Exam?
Jesus' message here is being packaged as a new 'covenant', to a First Century Jewish ear... but what's in it for us 21st Century goyim?
Is this our Ticket to Heaven (Make sure you've got one when you die? Because if you don't, God will be really sad... not to mention you?) Well, Jesus has so far said nothing whatsoever in this book about anything of the sort.
There is, of course, the Jewish (Pharisee) doctrine of the resurrection, which Jesus apparently shared, as presented in Daniel & the Maccabee books. (Once Jews had encountered persecution & martyrdom for their religion, this previously tacit issue had become a serious concern to them.)
Since this doctrine included a Judgement, to separate the martyrs from their persecutors, and reward them both accordingly... that too became a popular element of both Christianity and Islam.
Especially the part about Eternal Torment for those annoying friends of yours. This has been a powerful incentive for conversion... but has also worked to discredit the very idea of God. And so, over the years, it has been softened in the public mind to something like: ~"God doesn't care what you believe, so long as you've been A Good Person."
In so far as what you believe affects what you do and who you become, I expect that it does matter. But God is hardly in a rush to tell you about anything you don't care about, and there is plenty of time.
The crux of this all, as I see it, is that God is compassionate. Eternal punishment is not. Neither does it make sense for God to make "His" favor contingent on getting The Right Answer. If you want to know how things be, ask! And be prepared to find yourself answered in some way.
[Recently reading The Science of Diskworld, I picked up a handy phrase for a concept I've always found useful: "lies-to-children". A "teacher" is a "liar-to-children" because, after all, a simple, partial truth is a whole lot more use to a child than either being left in pure ignorance, or given a bewildering full explanation. And it's been a Quaker commonplace, from our beginnings, that "God is here to teach us 'Himself'". Just remember, while you're learning, that whatever you've learned may be subject to overwhelming attacks of Changing Perspective, with luck.]
So the main reason I can see, that we're still being advised to do as Jesus says here-- is that nothing else makes any sense. We really are part of the same Mind as everyone else; we can't harm them without harming ourselves; and there's no one but God worthy of our anger. You have a right to be angry at God... but if you are, which of you is more likely to be right?