In the case of High Lonesome, it might have been Dad who changed them because the movie came first. I think the underlying short story was In Victorio's Country.By whether he changed them or not there can be a number of reasons. There might be a similarity to another movie character who was around at the same time. A name used might LOOK good on paper but might not sound good when spoken. I'm thinking of Ches (Chester?) Lane in The Gift of Cochise. The producer of Hondo came to Dad and said something like, "John Wayne can't be a Chester!" Dad suggested the nickname "Hondo" because he'd also used it in One night Stand where a character was named "The Hondo Kid."
Those are practical reasons, beyond that a screenwriter has to make the story and characters THEIRS to do a good job of writing. Sometimes the writer is a sophisticated enough thinker to do that without needing to make an obvious change, sometimes not. But, to write a screenplay you have to be able to "play" all the parts, to get inside the characters and invent what they say and do. No novel contains enough material to feed you all this, especially since movies are always EXTERNAL to the characters, meaning they can only see the story from the outside of the characters. That forces the writer to change how the story is told, what scenes occur, etc. So, okay, imagine you're the writer, you have to create all those parts, and a character has got the name of the guy who stole away with your wife ... you've got to change it or you can't write it fairly. That's an extreme, and forgivable, example but it gets the point across even for cases where the reason is much stupider.
There's a lot of dumbass stuff that is done while adapting stories, but the reasons for a lot of it are remarkably similar to the legitimate changes. At the end of the day, the movie is not the book. The movie doesn't damage the book. The book is still there. Just to keep going on this theme for another moment. A lot of people will tell you the book is always better than the movie ... well, that's true because a book is PURE CODE, the alphabet, sentences used to program your individual imagination. It's a partnership with the writer to create an entertainment experience just for you. On the other hand, read Jaws (not a bad book), then see the movie ... because, it stripped the story to its essence and is SO much better. That said, read Close to Shore ... which is the true story of the shark attacks that inspired Jaws, it is an interesting 3d point of view on the story with more detail than either of the others.