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At this moment in The Godfather, the Corleone family is weakened by the missed hit on the Don and the fact that Luca is missing and they are trying to find him is another issue that could stack on their problems.
By informing the Corleone family of Luca's death, the Tattaglia sort of help them by relieving them from this issue.
Wouldn't it be a best move to let them in the dark and not let them know that Luca Brasi sleeps with the fishes?
By revealing that Luca is now dead it makes it clear that the Tattaglia's weren't fooled by apparent betrayal of the Corleone family by Luca Brasi.
Remember Luca was doing this on the instructions of the Don....showing that they weren't deceived is a show of strength and, at the same time, shows that they have taken one of the Corleone's major weapons off the table.
