They are on some vehicles, mostly diesel engines and industrial applications. They are closed systems as well. When used in diesel engines some of the benefits according to the WIX site are
Properly installed and maintained, coolant filters help to provide:
A clean, well-functioning engine cooling system.
Proper engine heat dissipation.
Engine efficiency through improved heat conductivity.
Method to introduce necessary Supplemental Coolant Additives (SCA's)
Plus with the filters that are treated with corrosion inhibitors, that would be a bonus as well as corrosion is one of the biggest problems in any cooling system