RULES on PostgreSQL

Hello all,

Forgive my ignorance. I'm trying to understand the RULES in PostgreSQL and
I have several questions. Why the rules if they are not part of SQL92 or
SQL3? Where do they come from? Are they from before the SQL history of
Postgres? Many things that can be done with RULES can be done with
TRIGGERS even where there is no FOR STATEMENT in the CREATE TRIGGER
statement of PostgreSQL. Are the rules in PostgreSQL, mainly, to implement
VIEWS, so they are some kind of intermediate step between the parser and
the optimizer?

TIA, Jesus.