This parameter is common to tune, but explaining what it does requires a lot of background about how queries are planned. That will be covered in Chapter 10, Query Optimization. Particularly, in earlier PostgreSQL versions, lowering this value from its default, for example, a reduction from 4.0 to 2.0, was a common technique.
It was used for making it more likely that the planner would use indexed queries instead of the alternative of a sequential scan. With the smarter planner in current versions, this is certainly not where you want to start tuning. You should prefer getting better statistics and setting the memory parameters as primary ways to influence the query planner.
