Layouts versus theme templates
Before Field Layout was added to core this functionality was provided in contrib by Display Suite. With these modules you can configure layouts in UI instead of theming them in node/block/paragraph templates. This is not about one versus the other, this is more about the skill level needed to change the layout. You can use both methods at the same time. However, when you install one of these modules they add a new layer to theming, so you need to adjust any existing node/block/paragraph templates which changed the field layout.