It's got to do with the "time criticality" of information.
If you pick up a terrorist and go through his cell phone directory and get a bunch of numbers, you'd like to be able to put taps on those numbers....before word gets out that those numbers have been comprimised.
If you wait even a day, word will get back through other terrorists in the cell that so-and-so was nabbed and "they got his cellphone".
OK .like a 'no-knock' drug warrant.