Linked from
The 29 pages that link to Reverse osmosis, each with the reason it gives.
DesalinationRelated: It is the leading membrane method for desalinating seawater and brackish water.
Water softeningRelated: It can remove hardness ions along with many other dissolved substances.
Maple syrupRelated: Some producers remove much of the sap's water before boiling.
ArsenateRelated: It can remove dissolved arsenate from drinking water along with other ions.