Do your hands and fingers feel weak when playing barre chords and scales?
Having your thumb in the upper, middle back of the neck when playing scales, barre chords and power chords can help a lot with feeling you’ve got a better, more powerful grip on the notes you’re playing.
Think of it like clamping down a piece of wood with a vice – you want the top half of the vice and the bottom half of the vice to line up.
If they were at different points on the wood, they probably wouldn’t hold it down as well.
The same goes for fretting – if your thumb and fingers don’t line up very well, how are you supposed to hold down the notes you want to play with strength and command?