To me, [Syn,Lin]thesia are very useful when YouTubers don't have time to transcribe the thing into proper sheet music. Hari Sivan[1] and Kyle Landry[2] do this and often times fans may work together to make a proper sheet[2].

Music theory helps piano players understand musical pieces better, simplifying the complexity of piano pieces and reducing the time needed to learn new pieces. It turns "put finger here, then this one here" into "play this major chord progression in the left while the right hand plays a melody starting on the first note of the major scale..." and so on. Even just understanding how to make different chords (major, minor, diminished, augmented, dominant, etc.) helps tremendously. It won't be learned from either [Syn,Lin]thesia or sheet music alone.

[1] Hari Sivan Rito Village Normal: https://www.youtube.com/watch?v=YdNUX6Z8rb0 => Synthesia: https://www.youtube.com/watch?v=Qdq_NJvbUD0

[2] Kyle Landry Normal: https://www.youtube.com/channel/UCk0UErv9b4Hn5ucNNjqD1UQ Synthesia: https://www.youtube.com/channel/UCdkSsVttkG60X5nC0CJ2Xjg Sheets: https://musescore.com/brycepoindexter