It lets you change page layout parameters in small steps over a range of values using options. It can set \textwidth appropriately for the main fount, and ensure that the text fits inside the printable area of a printer. An rmpage-formatted document can be typeset identically without rmpage after a single cut and paste operation. Local configuration can set defaults: for all documents; and by class, by printer, and by paper size. The geometry package is better if you want to set page layout parameters to particular measurements.