What is the purpose of the changes to Game of Thrones show?

I can immediately see that our opinions on this differ somewhat, so I'm not neccesarily expecting the thumbs up, but I'll try to outline some of the main points about adaptation: in this case, from the Book to the TV Show.

Firstly, you should know, the process of adapting original work is such a common part of TV/Movie making it is its own profession. It also has its own Oscar category, which this year 12 Years a Slave won.

Now the reason this film won, is precisely because it was not just the book played out on film. It was faithful, yes, and it communicated the themes of Northups' book eloquently; but it was also aware that it was a film, and not a book. This is the craft of screenwriting: the remediation of material from one medium to another, in recognition of the limits and advantages of its chosen medium.

There has never been a film made that was 100% faithful to its source material. Its simply an impossibility, it's beyond comprehension or the bounds of possibility. To condense a novel into a film, even a novelette, would be an exercise in futility. It would negate the point of its own existence, contributing nothing new to its culture.

Every remediation loses material (often to shorten its length in fitting running times/episode length) and also adds its own; costume is usually a big part of this, where the translation into a visual medium means elaboration is required.

Game of Thrones in particular is excellent at this, with its incredibly detailed costumes making Westeros seem rich with different cultures, and helping to flesh out the traditions further than was possible in the book: a chapter about embroidery would have stunk up the place, but its possible to communicate these details in film.

Both mediums are limited, and play to their strengths. Game of Thrones is already incredibly sprawling as it is, probably reaching the complexity ceiling for a TV audience. That's not to undermine TV audiences by the way; it's just a reality that there's only so many hours of TV people can be expected to sit through, and remain interested. Some can sit through entire seasons (I'm a Netflix Boxer myself), but a lot of the audience just wants to catch up with the show once a week when its gone, and then cherish the anticipation of it not being on for a year or so. It's part of the natural process of consumption.

So with all that in mind:

You're a screenwriter, you've been tasked to turn GoT into a TV series that can withold its own narrative (i.e you don't need to have read the novels to understand it) and it's on TV so its in seasons. To capture every single detail would take screentime running to about 60 seasons: you have the budget and probably will be able to hold an audience for maybe six-ten seasons. There's no point in being idealistic about it, the reality is you have to cut some elements out...

What do you Cut?

The GoT show is the current Showrunner and production teams answer to that very question. Yours will be different, but would it be better? if you think 'yes', I recomend getting on a screenwriting course, and seeing how it works out. You'll have to join a long queue in a competitive business, though...