It's so important because most seizures are fishing expeditions. A political rival can be targeted with searches to find something that can be used against them. This could be anything from a misplaced comment in a chat log: 'I am going to kill that guy', 'man, someone should give that policeman the same beating he gave the protestor'. Likewise, a browser caches all manner of material, you can easily find your way into the wrong forum topic, or some vandal tweets you a link to child porn, or even regular porn that would be illegal in a certain jurisdiction.
I now see half way through posting that many other reasons have already been listed, so I'll end it at that. See the plight of Jacob Applebaum who works on the Torproject for a very specific example. I think it got to the stage where he had to replace his electronics when he travels because he couldn't trust what was returned, thats if he gets the equipment back at all from customs, or purchases fresh equipment in foreign countries to know they are 'clean'. I think he weighs is equipment too.
I now see half way through posting that many other reasons have already been listed, so I'll end it at that. See the plight of Jacob Applebaum who works on the Torproject for a very specific example. I think it got to the stage where he had to replace his electronics when he travels because he couldn't trust what was returned, thats if he gets the equipment back at all from customs, or purchases fresh equipment in foreign countries to know they are 'clean'. I think he weighs is equipment too.