Where Block Drupal module beats Layout Builder
Drupal core ships Block and Layout Builder modules.
- Block module is in business of placing blocks.
- Layout Builder module is in business of placing blocks.
But...
Block placements are decentralized: Each block placement is a config entity. Several modules can work together to place blocks on the same page. In other words, a module providing a block placement, doesn't know if a different module is also providing a placement on the same page or even on the same region.
while...
Layout Builder placements are centralized: All the block placements are living in a single object (e.g. as 3rd-party settings of the entity view display). This means that the underlying object should know that the modules providing the block exists. As an effect, the object providing the block placements has a hard dependency on modules providing the blocks. Third-parties are not able to hook-in by themselves, they should be dependencies of the module that provides the object providing the placements.
This is, probably, the only advantage of Block module over Layout Builder. Its decentralized nature.