I usually do this to protect a big fleet in the SZ directly behind the blocker. Or as a (usually) 1-turn preventive measure against invasions.
Example: Japan wants to delay the USA from reaching a critical zone during a ‘KJF’. For the sake of simplicity, lets assume the USA wants to reach SZ6 with a much bigger fleet than the IJN.
Put a blocker in SZ16, now the USA can only reach SZ6 with air -not enough to be of any problem for the IJN. IF a Japanese counterstrike from the IJN + IJAF is a concern for the USN, this blocking tactic may work for a couple of turns in a row, even indefinately…
This is precisely what I do as Japan, particularly mid-game if the USN is holed up in Hawaii (which they usually are) and the IJN is split into several task forces operating in different SZs. It lets Japan use SZ 6 as a safe placement to mobilize new naval units. The DD block is critical in keeping the USN threat to a minimum while preserving Japan’s ability to split its fleet to deal with multiple threats (or to consolidate disparate naval builds) - not to mention keeping the US honest and unable to send in a few subs to cause havoc around the Japanese home islands (a common “tactic” or sometimes “strategy” I’ve encountered from US players looking to stay engaged while they focus elsewhere (usually Europe)).