Is there a reason for using PostGIS functions such as
ST_Length when there are already functions/operators for doing the same thing in PostgreSQL (
I think the linked answer is good.
But to clarify.
The standard PostgreSQL spatial functions work on the PostgreSQL geometric types. While the PostGIS ones work on the PostGIS geometry types.
PostgreSQL offers a few functions to deal with spatial operators:
It does not offer the full set that PostGIS does:
So if you will ever need a function that is not supported by PostgreSQL, you should go with the PostGIS geometry type, which allows for more spatial functionality.