In all honesty, I feel like many of them weren't necessary but for aesthetic and film trope reasons, to the point I believe he completely shifted a good chunk of the philosophical and theological implications of the books.
I recall Christopher Tolkien had issues with the movies as he felt they glorified battle, which was against what he felt his father intended.
There is a large theme of pacifism and the tragedy or war in Tolkien's magnum Opus. But unlike Tolkien who was able to get past the battles as quickly as possible and take a more objective look at them as a writer, Jackson knew action was one of the main draws to a mainstream audience, and that he couldn't gloss over things the way Tolkien sometimes did.
There are three major themes forgotten or ignored by the movies that were in the books.
Firstly is the stance of war being a last resort and not something to glorify, which is still somewhat present in the fact Frodo never got over his trauma and it eventually forced him to leave Middle Earth and the suffering we see people go through due to the war, Jackson made sure to emphasise the horrors of war in Two Towers especially. However he couldn't skimp on the action scenes, they were one of the biggest draws to an audience, and a chunk of each book that is plot necessary occurs around these battles.
Secondly is Monarchism. Tolkien was a bit of a romanticist and called upon old fashioned ideas of the monarch as having a connection to the realm, hence why the line of Stewards see's Gondor decline despite competence, they don't have the figurative and literal healing hands of a king. For modern audiences, especially American modern audiences, Aragorn from the books and this message is less palatable. In the books Aragorn has been working towards becoming king since he was in his twenties (about half a century ago in the books), he carries around the broken Narsil, gets Anduril before leaving Rivendell, introduces himself as King whenever he meets another noble etc. Aragorn of the movies is an underdog who is unsure about wether he should become king and shameful of his heritage. Hence why in the books he's called the heir to Elendil, and in the movies the heir to Isildur. This underdog and reluctant king is much more acceptable to a mainstream audience in the modern day, however Jackson still kept the King being a better ruler then others segment, just hidden by continually reminding people of a bad king (Isildur) so as to make the medieval take on monarchy less clear to an audience that is largely liberal, meritocratic and democratic.
Finally and most famously, the idea that the battle against evil never truly ends. This is symbolised by the scouring of the Shire, the Hobbits couldn't just go home and enjoy their lives with the dark lord gone. It's also seen in how Tolkien believed that there would be another dark lord when he started, then aborted, a sequel to Lord of the Rings about people who worshipped Sauron forming cults across the Reunited Kingdom. In the movies it's far more clear cut. The bad guy is gone, most of us get happy endings, except for Frodo who is traumatised by the war and violence.
I'm going to be honest with you, Return of the King had almost too many endings, if the Scouring of the Shire had been added…well, that may have been a step too far. And you can't leave a massive trilogy like that on an anti-climatic note like that. Tolkien could, because it fit the themes and his writing allowed for it to feel as though it was part of the war, as the books were able to show the wider scope of the conflict. The movies had to monofocus, and the scouring of the Shire wouldn't have worked without the buildup and larger picture the books could afford.
I've gone a little into the character changes of people like Gimili, and characters and scenes skipped out like Tom Bombadil and the Barrow Wight, and I could go far more in depth. But I feel Jackson was handling a different circumstance, unable to show as much, having to give audiences something closer to what he knew they'd find palatable. He found an awesome balance between book loyalty, filming constraints and appeasing and audience in my opinion.
