postgresql - Purpose of geography_columns and geometry_columns views in PostGIS?

Can anyone tell me what is the purpose of geography_columns view and geometry_columns view in a PostGIS database?

My geography_columns view is empty so I can't see what the purpose of it is.

Answer

geometry_columns is part of the Simple Features specification for OGC and ISO 19125-2:2004 Standards compliance. geography_columns is a non-Standard variation to support the geography type.

From http://postgis.net:

PostGIS follows the Open Geospatial Consortium’s “Simple Features for SQL Specification”