Frigates were a core component of a fleet, essentially the high mobility anti submarine class, and played the screening and scouting role vs subs.
it can be described as an "anti-submarine escort vessel that was larger than a corvette (based on a mercantile design), while smaller than a destroyer." ~check link
Im not super experienced with the game but deploying own submarines feels like the main counter against enemy submarines, which i find a bit lacking in dynamic. (especially for the mid-low levels). So this class feels like a healthy addition.
https://en.wikipedia.org/wiki/Frigate#:~:text=unprotected cruisers".-,Modern%20era,-%5Bedit%5D