Open source developers sometimes have a hard time figuring out what feature to focus on to generate the greatest value for end users. As a result, they will often default to performance. Performance is the one feature that every user approves of. The software will keep on doing all the same cool stuff, only faster.
Constraints trigger basic relationships and a look at more complex use cases like deferred constraints, and full table-level data structures.
How to guard spatial data quality using constraint triggers and spatial relationships.
Constraints are used to ensure that data in the database reflects the assumptions of the data model. Paul Ramsey demonstrates simple and complex constraints that spatial database users like to enforce on the geometry data.
The raster functionality in PostGIS has been part of the main extension since it was introduced. When PostGIS 3 is released, if you want raster functionality you will need to install both the core postgis extension, and also the postgis_raster extension.