In PostgreSQL there are two different ways to perform partitioning - using rules and using triggers. One of the usual reasons why a partitioningu is used, is a performance, so lets analyze differences between these two ways regarding performance.
Just as in case of PGMon project, when collecting this kind of data there may be huge amounts of data, but these data go out quite quickly - in most cases it's not necessary to work with data older that 1 month. There are two consequences of the quick growth - decrease of performance and problems with maintenance (backup and removal of old data).
The amount of data collected by PGMon may be quite significant, and it may cause serious problems regarding their processing (the more data the slower queries) and maintenance (collected data become stale quite quickly, and deleting large amounts of data may cause problems in PostgreSQL). Partitioning represents relatively effective solution of these problems.