For a Magistrate to issue a search warrant the police must supply reasons for their requiring it. They cannot simply go "fishing." They have to suspect that an indictable offence has been committed and that evidence may exist on the premises which will assist in the prosecution of that offence. They have to convince the Magistrate that without the warrant the search would not be possible, why it is necessary, what they are looking for and why they suspect it is there (this last requirement may need them to produce evidence of their information and the reliability of that evidence). The request must have been authorised by an office of inspector rank or above officer requesting the warrant will have to provide all this information under oath.
A search warrant is a powerful tool and a considerable encroachment on the occupiers privacy. They are not issued lightly.