In the sim, the Range/RRF stat is essentially used in the determining the effectiveness of a fielder to reach a ball that is put in play. In the event tree of the AB, once the sim determines that there is a non-HR hit, it will use the Range/RRF stats to determine if a fielder turns that hit into an out (thus, granting the fielder a + play) or if an out is turned into a hit (thus, granting the fielder a - play).
Essentially, the higher the Range/RRF, the more likely that your players will receive + plays and turn hits into outs. The lower the Range/RRF, the more likely that your players will receive - plays and turn outs into hits.
WIS uses RRF to quantify their "Range" as it is about the only way they can compare players across all eras as defensive stats dating back to 1885 are tough to come by. So while it may not be perfect, it is one of the only ways that WIS can normalize defense across all era.