This was the result of a no-knock warrant: http://en.wikipedia.org/wiki/No-knock_warrant , which allows police to enter a home without prior warning, on the theory that giving notice will allow people inside to destroy evidence. They are almost uniquely a product of the drug war. One can imagine very few situations outside of drugs were evidence could be so easily destroyed as to justify a no-knock warrant.
I see no evidence of this.
In addition to preserving evidence, they are also issue when it's believe there could be an issue of officer safety. (E.g., someone armed & dangerous.)