They were used to inactivate airfields becuase, while they did not immediately explode, the ground forces were forced to clsoe airfields because they did not know when the delayed fusing would go off.
I read in the unit history (
http://www.schifferbooks.com/newschiffer/book_template.php?isbn=0887407501 ) for the 49th FG that some of the bomb delays the USAAF where using were as long as 72 hours, not sure what the US Navy, Marines and other allies were using. But it really played havoc ( hellish even ) on the crews responsible for the repair of the airfields and anyone else on or around the airfields.
If I recall though, there fuses where not on the cluster bombs, but where used on 250lbs bombs and up. The bombs would bury themselves without much evidence to mark there passing because the tropical rains keeping the ground somewhat soft and often muddy making it very difficult to locate and dispose of them. According to that book, this info on the true effectiveness of these long delay fuses on those bombs came to be realised after the war was over, through some surviving Japanese documents. Not just with material damage and death, but significant psychological harm.