That the exact location of where rockets land was always treated as sensitive information everywhere to make it hard for the other side to adjust their targeting? During WW2 the British government went as far as coordinate fake news about where the V2 rockets landed with the media to make it harder for the Germans to hit anything.
A major difference here is that during WW2 the rockets were unguided and fully relied on fuel-weight calculations. These ballistic missiles are GPS guided and don't suffer from all of the unknown factors.
There is certainly a case to be made for secrecy, though it's more likely they don't want to let Iran know whether F35's, tanks or other military equipment were hit. Israel already claimed they "intercepted the majority of projectiles" but the videos don't really corroborate that. Similar to the Hamas "limited ground operation" last year israel tries to hide all evidence and then crafts their own story of what happened.
Because it's feedback on how effective their targeting has been when confronted with whatever electronic warfare and misdirection Israel was using to defend themselves.
That sort of information might let the attacker make adjustments to be more accurate next time