Those overwrite your mirrorlist.
Assuming you dont have a mirrorlist only made up of 1 (or a few) all broken mirrors … then -f should be fine to rank from your custom list.
If you dont have a custom mirrorlist … then -f will query all of them.
(well it would … but if you feed it an integer like 5 … well thats the number it will query)
g also has the distinction of focusing on what is nearby - not what is fastest.
So … they all have different uses … but -f makes sense as a default.