Most commonly because you'd like to use JDBC to connect to your local Postgres server without using password auth. Very quickly you'll realise Unix sockets aren't supported in Java, so you can't use peer authentication.
First, ensure your /var/lib/pgsql/pg_hba.conf uses ident for TCP/IP connections:
# IPv4 local connections:
host    all             all             127.0.0.1/32            ident