Its to grab any iron particles & stop them from going in your engines injection pump
Not familiar with Westerbeke
12b2 to be fair.
A good idea frankly IMO.
I add neodymium magnets to my drain plugs & filters on any fuel or oil
system despite the fact the filters should catch the iron particles anyway.
They do hold some particles on sump plugs IME. I've never cut open the oil
filters to check, just shift magnets to new filter .
Dont know if it makes any difference to engine
life but I hope so & it makes me feel better