When it was announced that "Wicked" would be split into two movies, many, myself included, wondered how they would justify that choice. This choice was only made more confusing when the runtime of part one was longer than Act I of the musical. When "Wicked: Part 1" was great, it looked like those worries were unfounded. Then "For Good" was released.
"For Good" added two original songs: one for Elphaba (Cynthia Erivo) and one for Glinda (Ariana Grande). Despite how great the performers are, the songs are forgettable and don't add anything to the story. There are a few other questionable song choices made in "For Good". In the musical, "Wonderful" is a duet between The Wizard and Elphaba. In the movie, Galinda joins in. This takes Something away from The Wizard, and changes his character. He uses his slick-talking ways to get Elphaba on his side (even if it's just for a moment). The movie gives this moment to Galinda, and it relies on their relationship as the reason Elphaba almost joins them.
Subtitling your movie after the best song of Act II means you'd better nail that song. Musically, they do, dramatically, they don't. On the music side, Erivo and Grande deliver a powerful rendition of the song. By itself, the song works, but in the context of the movie, it gets made weaker. In the musical, Elphaba and Glinda meet twice in Act II: the first is a confrontation, the second is when they sing "For Good." This song is about the two witches accepting that, even though they are on opposite sides of this conflict, they both treasure their friendship and how it has shaped who they are now. This is a more powerful message, given that their last meeting ended in a fight. In the movie, they have had multiple interactions in which they have broached the topic of their relationship. It takes a lot of the emotion out of that song.