I have read the Harry Potter books, and am still reading. I have also watched the movies. Whereas the first film is rather close to the book, except for a few very minor scenes, the second is less consistent with the book. The later films are really different.
What was the point of making up new scenes, different from the ones in the books?
The question is: why did this happen and what did it accomplish?
Edit: In the first book the dragon of Hagrid's is brought to the roof of the tallest tower and handed over, Dumbledore didn't even know, whereas in the film it was Dumbledore who somehow knew and advised Hagrid to give it to Percy. In the first book Harry was in the zoo with Dudley and a friend of his whereas in the film Dudley's friend was missing. Even these minor changes are strange and out of logic. As for the other things: a lot of conversations which are relevant to the plot are cut out, a lot of quiddich practices and games are cut out, interpersonal relationship scenes are missing. In the fourth book Dobby gave Harry the jillyweed and there were a lot more things which were changed in the film. In the fifth book when Moody's company was talking Harry to the Order of Phoenix HQ they used a disillusionment charm and the flight was good, there was no battle on brooms whereas in the film they used Polyjuice potion and they fought their way to the HQ. This can be a long list but all I say is, there is no logic behind these changes and actually nothing good in them too, they really change the way things actually went in the book.