Police ask a judge for a search warrant when they want to look for specific evidence at a specific location. For example, if a police officer believes you have a stolen computer in your car trunk and you do not permit him to open the trunk, he will need to get a search warrant to look inside. A police officer must demonstrate probable cause of criminal activity to establish grounds for a warrant.
A search warrant applies only to the specific location and type of evidence described in the document. To search for additional evidence or to search another location, a new warrant is needed.